I think you need to tell A that sharing this feedback with you won’t help B change, and that they need to address B directly or talk to their supervisor.
You can also say that sharing this feedback with you is putting you in an uncomfortable position, as you are friends with both of them, and you need it to stop. It’s perfectly okay to validate A’s complaints (“I understand why you feel the way you do”) so that A doesn’t feel like you are dismissing them. But that doesn’t mean you have to be in the middle.
Having spent many years in corporate life, I can tell you that one of the biggest blockers to people improving is that no one tells them there is a problem to begin with. Person B may have no idea they’re underperforming. And to be fair, I can’t tell from this whether their supervisor would even agree that B is underperforming; B may be doing just fine from management’s perspective, in which case A needs to let it go.