A seeks out a new peer group, iincreasing the odds that A does end up leaving the group. So B is engaging in classic self-defeating behaviour ... unless, of course, the peer pressure succeeds.

Which it usually does. In the ancestral environment, opportunities for seeking out a new peer group were quite limited, so our brains don't quite realize they can do it; they're still quite biased towards keeping the existing group happy.

If this weren't the case, it wouldn't be so necessary for wealth, self-help, PUA, and other gurus to harp on the importance of doing it, and of being prepared for a negative response from your existing peer group.