<human>: Can you provide a brief overview of the text?

Hey guys, long time reader, first time poster.

Here we go. My sister is going through a tough situation with her long time SO (6 years +, bought a house together). There is still love between them, but they decided to live apart for a month to think about their future. 

They used to spend so much time together that they kinda drifted away from their respective friends. So for the past month me and my gf took the sis out, trying to restore her social life a bit and change her mind.  This is where my best mate comes in. Keep in mind that my sister smiles a lot, and of course wanted to make a good impression, as all of us do when we meet new people. So this friend of mine is a little clueless when it comes to social signs, he mistook her happy personnality for attraction. I even heard him disparaging my sis's bf during the night. Now he thinks he has a chance to make a move for her, while her couple is on the rocks. He calls her knowing she can't say no at the moment, after all she is enjoying the attention she has been missing, good for her. But honestly I wasn't ready for this behavior from my friend, at this tough time, I see him in a new light now, a bit of a creep really. 

This guy is the best friend a man can have, and I don't understand why he can't see that he's taking advantage of her at a time where she needs friends, not sexual partners. 

As the older brother, I support my sister whatever she chooses for her future. But as for my mate, I'm wondering if I should just go to him and ask "Do you really want a girlfriend who is only your girlfriend because she has manipulated by you?" Or should I just let him make his mistake?
<bot>: Should I tell my lifelong what everyone is thinking, ie that now is not the good time for a relationship with my sis, and that he is taking advantage of a vulnerable human being that is very precious to me?