i hate it so much

so i am with my fiance before we got together he was with a girl, they had an accident and she got pregnant. then she decided to have an abortion and let him know. flash forward to now, i get a message on IG from her saying the love of my life/bf (she doesn't know about the engagement obviously) abandoned his second child. well i was with him when i got the message and was like "you don't have any more kids right? " and he said no so i said then whats this and showed him the message. he explained what happened and how she must have lied. but she was also sleeping around at the time so it might not be his. im all for him stepping up financially if its his but unfortunately for the sake of me, the baby, the mom and him i think its best if thats it. like how could you lie like that and fuck someones life up