She's abusing their friendship and He's allowing it. He could say No at any time and hasn't done it. There's a bond here that's not healthy for either of them. If you were to marry him it would continue. He must break his ties with this girl. Because, otherwise, he'll be trying to handle the emotional connections of her, himself, and with you. He's got too much on his plate.
You can refer her to someone to talk to but I don't think she'll listen to you because she feels she can trust Him.
Best Of Luck
Ocean