Some would label that codepenent behavior. Making decisons for others in an effort to control or predict the outcome of something. There can be an equal amount of drama on the flip side of things. I won't paint the whole scenario, but basically... guy gets pissed at girl for getting him kicked out of band, guy loses both, hates everybody. Yeah, the band might (or might not) be free of most of that drama, but if they somehow remain friends it could get real messy, and regardless, all involved can say that the right thing wasn't done. Lots of drama potential there. Guitarist didn't do anything to warrant being kicked out of the band. GF did. To me it would be clear that either she doesn't come to gigs any more, and if they disagree to that, then he can't be in the band.
I believe that's taking the high road, and there's no room for argument or complaint.
Ya know... after thinking about this and writing what I just wrote, I actually think the high road would be to talk to the girl, make sure it never happens again, and move on. People do stupid things. If they repeat them, then there's an issue. If it's a 1 time deal there's nothing to be gained by getting rid of anybody. Unless you have other reasons you're not disclosing.