What Should my sister do (boyfriend wants a transvestite)?

So my sister discovered that the boyfriend she has been living with for two years is basically gay. He left his facebook open and she discovered messages in which he bad mouthed her and asked to see a past girlfriend for a date. The worst messages were those asking about a transvestite escort which he has apparently been with before. He basically wants someone who looks like a girl but has a penis. She left his house but has yet to confront him. The thing is she's not a jealous overbearing girlfriend so its not like she's forced him to this or anything and she actually thought she might marry the guy one day. In my meetings with him he seemed entirely straight and normal and he's in his mid 20s so this cannot just be some sort of weird phase. Do you consider him gay and what should he do?