So: Friend A and Friend B keep sleeping together. Friend A wants a relationship with Friend B; Friend B is uninterested in this, has never made any move towards a relationship or suggested this was a possibility, and has made it clear it's just about sex for them, but Friend A keeps convincing themselves that a relationship is a possibility.

Friend A then gets very upset after each time they sleep together and Friend B hasn't changed their mind, and leans heavily on you and Friend C for emotional support. You and Friend C have repeatedly comforted Friend A and strongly advised them that they should stop sleeping with Friend B because it upsets them, that Friend B won't change their mind, and given them inspirational pep talks to try and boost their self-esteem, which seems to be the major problem here. Additionally, Friend A has also been involved with quite a few other acquaintances of your and Friend C's, most of which have resulted in similar situations whereby Friend A wants more than sex but doesn't receive it, and has also required support for all of these occasions too. You have suggested Friend A see a counsellor to help with the esteem stuff, but this is apparently not an option.

What do you and Friend C do now it's happened again? Do you give Friend A short shrift because they're not listening to you and keep making the same mistake? Do you refuse to discuss the situation with them at all because you're fed up of it? Do you talk to Friend B about it (Friend B is aware that Friend A gets upset easily and has taken to asking them before hook-ups if they will be OK with it)? Or do you keep repeating yourself and hoping that Friend A will eventually listen?

(This is genuinely about a friend and not about me :D Wouldn't consider Friend A to be a close friend or anything - we only talk much when they're having romantic issues - but they're part of my friendship group.)