I'm going to date my best friend's long term girlfriend a week after they broke up
Dating your best friend's long-term girlfriend after they broke makes you look like a bad friend.
I need to fill the void for loneliness for my friend
If you do that and date the ex-girlfriend it will look really bad on you. Your friend will probably want to beat you up or worst, end the friendship.
I think it will be OK.
Trust me, your friend will not like this. If you want to stay friends, you will stay away from his ex.