Gay or straight a cheater is a cheater. Her preference has nothing to do with the discussion. Right? So she has problems with her mate. Yeah true it is none of your business. HOWEVER if she comes to you and asks you for advice then give it. No one should feel like they have to stay with a cheater. Be her friend give her your true opinion once and let her decide if she wants to take your advice. As to the Depression being with someone who is hurting her will not help her get over any serious problems she might have.