I've got this currently, one of our guitarists has little awareness of his volume at times.
I've spoken to him about it in the past and the issue is most of the band think it's ok as they like playing loud, whereby I like looking after my hearing and not pissing off the audience, so it wasn't really addressed given it was a 4 vs 1 type situation.
We even had a sound engineer come and see a rehearsal so we can get an independent opinion to try and force the point. The first thing he said after listening to one song was "First of all the guitars are way too loud, they are actually drowning out the drums. They need to come down"
My 'shocked' reaction as below
The thing with our band it's not an ego thing as he's a really nice guy, I'd put it simply down to inexperience and he's not sorted his effect levels correctly, which is then making our second guitarist turn up. So it's fine with gentle chats and nods about the songs in question to him and them both during rehearsals and gigs, when my ears are screaming from the amp volume.
He's a nice guy and I like him so I don't want to be an arse about the matter as it feels unintentional. Hoping it'll sink in at some point, could see the bass player looking at him like "wtf?" last night during one of the songs in question though.