I think it's poor planning, deliberate or otherwise, put some shorts on. : )
Nooooo and I'm pretty sure most people don't do that.
It depends! I mean if that happens suddenly, that's weird & it would be a shocking moment cause it's not appropriate but it happens out of control so what we should do now? If it's something that happened all of the sudden, then we better not make a big deal out of it cause there's no big deal, we all are humans & we all made by a body & wear underpants, so not a big deal.
But if you've been in your underwear on purpose till your friend shows up, & it's been so obvious also for your friend to recognize it like you are siting with your underpants on & waiting for him, that's totally weird, & not really fine cause i'd agree with (Tiger) here, no class & kind of would makes the opposite person feel awkward, uncomfortable & surely curious like what the heck!
Your friend is male, if that friend of yours had been female, then I would say it's so rude & so disrespectful because behaving with a lady however the maturity & closeness of the friendship be, is different with a close guy friend in these topics. That's how I think.