I picked "Y'all need a lot more practice" just so I could see the poll results!
But in reality, the answer would depend on HOW subpar they were. They might really just need a lot more practice, they might just need to polish it up a bit or they might really just sound like ass with no apparent hope for improvement.
In the room I might say something like, "not bad" but I'd do the Roll Eyes of Doom of only my friend could see it. If he started to nut up right then I'd just say, "
Let's talk about it later"
If he's so addicted to the post-show high that he's gonna pout or throw a fit about that, then I'd probably stop going to his shows until he changed his attitude. when he caught on and asked me why I didn't go to his shows anymore, I'd tell him it's because he can't take constructive criticism and I don't want to hurt his feelings.
It also depends on the look on his face when he asks you how it was. If he has that deluded look like he thinks it was good, you'd have to be gentle at least right then. If he's got that downcast look that tells you he's just hoping it wasn't as bad as he thinks it is, then you can be more honest.
Some people think they can make the band by themselves. That can work if everybody else is at last competent and getting at least a decent tone. But if someone really sucks or is even good but just has suck tone, you have to say something. Surprisingly, a lot of decent players think their gear sounds good just because it's their gear. Crate players are susceptible to this because their shit can sou nd pretty good in the bedroom but it doesn't cut through the mix live, or is being pushed to th point that it sounds like ass if you can hear it.
To be fair to Crate, their Blue Voodoo amps can sound pretty damn good for metal with the right tubes. But it's the $400 half-stacks that make their rep bad.