Opinion Please!!!

Okay I need opinions.

My husband is in a situation where him and this girl messed around and she ended up pregnant . Well long story short is she didn’t contact my husband when she was pregnant she was cheating on her man . So after 3 years my husband gets papers in the mail saying he’s getting tested for the child . Remind you that she name the child after the other man and he’s been in the child life for the past 4 years and now that they aren’t together she wants my husband to step in . This whole time the other man has been in that child life . What do y’all think the best thing to do ?