Long story, but what would you do?

My sister has been seeing her chap for 4.5 years now. Hes an ex Ford technician and has many many contacts in the Auto industry, but put that aside for one minute. He has been good in the past to me like helping me reshell me car, get me parts etc etc etc.
My sister has always had her suspicions about him cheating. She has now found out that he has got about 5 girls on the go, and 4 different kids by these women. One has has been seeing 13 years, one for 9 years and my sister for 4.5 years and he has managed to juggle these around and get away with it.
My sister hates him, me mum hates him and as you could imagine the reste of me family are pretty pissed about it all.
In light of this, do you think I should not remain friens with him or not. My sister says its up to me, but I can tell she aint happy. On the other side of the coin, I broke down yesterday and he came to rescue me and helped me put me car right. (starter motor and battery).
In your honest opinon what shud i do?