Sounds like you need to hug it out....just saying...

As a belayer, I know I get VERY nervous when the leader is sketching at/just above their only piece of gear. Having seen someone rip gear and deck - on more than one occasion - just makes this worse. But there's not much you can do as the belayer, except offer encouragement, and maybe suggest as nicely and constructively as you can manage that they do something other than bomb off.

From your account, it sounds like the leader knew that the piece was marginal. That's good. I suspect there are too many folks taking the sharp end today that treat trad gear like bolts. Call me conservative and old fashioned, but I still believe any single placement can fail, and I want a minimum of two pieces between me and a bad landing.

Mike makes an excellent point. Redundancy (i.e., if this piece of gear fails I'll die so I should back it up) seems to be something that everyone is aware of, but many people forget about it when placing protection.

Was the marginal piece the first piece? If so I def see the need for a fairly intense discussion.
If it wasn't the first piece, how far away/down was the last good piece?

Yell away dude, as far as I'm concerned, I'd want my belayer to yell at me if I'm doing something unsafe/stupid. And keep yelling the fear-pump makes people do stupid things.

Are ya comin out to the wasatch this winter, btw?

So there are different acceptable ways to yell what to do, to a climbing party next to you that you never met before compared to your S.O. of 10 years? I can see being much more vocal with your SO.

and yes i agree w/ Mike about doubling on gear.

Rob i may see u out there.

Geez, Talus how are we gonna get a really contentious thread going if you are so amiable?

I never yell at a leader while they are in the process of leading. If the situation is difficult, there is plenty of internal turmoil for them to deal with as it is, and adding another emotional element to the mix can only make things worse, in my opinion. Yelling at them afterwards is an option, but has more to do with who each of you is and what your relationship is.

Staying calm and operating as rationally as possible in a fix is one of the central challenges of trad climbing, and everything the second can do to promote that state of calmness is going to help. So, no matter how bad the situation is and no matter how badly the leader is fucking up, I always try to speak in a calm firm tone, to be, in a sense, the kind of internal voice one would hope the leader had available and may yet regain.

If this doesn't help, escalating to yelling will only add further static to a brain that is already overwhelmed. Basically, you now have two people who are freaking out, and that isn't going to help the leader extricate themselves from the mess they're in, and may even end up promoting rash decisions.

I must say that in the situation John describes, with a leader falling or taking on a marginal piece with the prospect of a groundfall, when they clearly had the juice to downclimb, I would probably avoid climbing with that person again.

It depends (TM). I'm missing a few things from your original post:
-- was this the first piece, or were there others (and were they good) below?
-- were there no other options for gear?
-- did the climber recognize it wasn't a good piece?
-- is the person a new leader, or (semi-)experienced?
-- was the person "in the weeds" or just being lazy and/or stupid?
-- were you yelling due to distance or anger?
-- did the climber somehow expect you to help him/her?

It still depends. Yelling may break through to a person so far in the weeds that nothing else will. Most of the time, like Rich said, uber-clam-calm is most helpful. Yelling may also completely eclipse the informational content of the message, and depends on both the relationship and the person involved.

As far as the future goes, if this was a new leader, it's a learning experience and if s/he understands that, you can move on. If it's an experienced person who really understands that they put themselves AND YOU in a super bad spot, and understands what led to that place, and that there were other better choices to be made, is having an epiphany about themselves and their climbing, you can get past it. If it's an "experienced" person who refuses to admit danger and/or wrongdoing, never again.

There are all kids of subtlety to partnership, truly. One of them is a duty to not expose your partner, unduly, to trauma, like the kind that will keep you up at night or send you to a psychiatrist. I am both guilty of this (Dana, and everyone who had to carry me out) and have a giant chip on my shoulder about it, having been on the receiving end of suicide-by-highway. I get very, very angry at people whose carelessness exposes me to unwilling participation in life's really ugly side, like, say, a body sticking out of a windshield. I get very angry at runners/cyclists who dress in black, black and more black, then dart out in front of you, because I really, really don't want to hit them and then carry that guilt. Sorry for rambling, but I think you get the idea: in your place I'd be angry at the potential exposure to your partner's groundfall.

I agree with rg@ofmc that there are more effective ways to communicate with someone than yelling. Yes, yelling is useful to attract someone's attention if they're far away, but yelling usually increases the emotion of a situation and interferes with comprehension and retention.

For example, yelling at someone in a public place usually embarrasses them such that they more focus on the embarrassment and less on what they did wrong. Basically, you're calling them out in public that they're stupid. We all make mistakes, the important aspect is to create an environment that's conducive to learning from that mistake.

As other folks have mentioned, climbing is a partnership. If your partner chose to ignore your advice and continues to do so, you probably need to find another partner.

Impossible to objectively evaluate without hearing the other side of the story.