Is it okay for my boyfriend to be really good friends with a female?

Okay my boyfriend is friends with his cousin's ex-wife. My boyfriend thinks a lot of her. To him she's a really good person and he considers her like a sister to him. In my boyfriend's mid 20 he lives with his cousin and his cousin's wife for a few months. Then he moved out. Well my bf's cousin separated from his wife for a short time. Well during that time my bf went through some financial problems his cousin's wife at the time said he could live with her till he gets back on his feet. Well of course that made my bf's cousin want to move back home. Well things didn't work out for my bf's cousin and wife they divorced. My bf remains really good friends with his cousin's wife. My bf says nothing ever happened between the two. I do believe him. But I know how women can be. She may of wanted to help him out when he was going through his financial problems. But also make my bf's cousin jealous at the same time. My bf swears nothing has ever happened between the two. But he says a lot of friends and family questioned their friendship.

I have yet to meet the girl. My question is can a guy be really good friends with a female. When he has a girlfriend?