How do I know if my mates ex girlfriend likes me?

sounds 2 me that she likes you or maybe is up for fun, the boyfriend/your mate sounds like an asshole to me she deserves better if he is horrible to her like that when her child is in the house, they pick up everything. only you and her can be the ones to decide what happens but if you are not interested in a relationship and becoming a father figure, (that may be what she wants) then I'd make a wise decision about it! Good luck