Im engaged to my ex bestfriends baby daddy?

ok so about a yr and a half ago my best friend at the time introduced me to my soon to be husband ( in 3days) he had a crush on her at the time but she only saw him as a friend. well me and him started hanging out a bit more and i started having feelings for him. i tried to not show it to him so he had no idea. however her beeing my bff i of course told her what i was feeling. after i told her i noticed she would get jealous when he would flirt around me ( even though she swore on our friendship and on god she didnt have feelings for him) and she came on to him one day at work ( they worked together at pizza hut) well since he thought he didnt have a chance with me bcuz i would never flirt back he slept with her. well i found out one day because i decided to let him know how i felt and his face just totally went blank and spilled what had happened between them. i was absolutely furious and upset at her because she knew i liked him alot. well we had a huge fight she denied everything and once again swore on our friendship well stopped being friends and about 3months after me and him started dating. well 2months into that we find out she is pregnant however he asks if its his and she says no! we continue with our lifes but somewhere inside i think she's lying well the baby is born she looks exactly like my fiance and the mama admits its his. im absolutely shaken up in tears because i feel guilty for some reason. now he wants custody of this baby because well he's the father and she wont give it to him because of me. which makes me feel even more guilty. should i be or not help??