Overthinking paternity

Okay so almost 2 years ago while my SO and I were together a girl came out of nowhere claiming that her 2 year old daughter belonged to him. She said she wasnt looking for money but just wanted her daughter to know her father. He was willing, wanting and ready to take full responsibility. However, before getting involved he asked her for a DNA test to make sure she really was his.

(They were fresh out of relationships when they got together for not even 2 months) After finding out she was pregnant she originally claimed her ex was the father, he has been involved ever since and was put on the birth certificate and as far as he knows that little girl belongs to him.

Anyways, once my boyfriend asked her for a paternity test she got incredibly angry, told him to forget about it and that she would be blocking his number. He hasn't heard from her since. But ever since then I can't help but think about it. We just assumed if she got so defensive that maybe she knows it's not his. But I've done some research and the timing would add up to the time they were seeing each other. However my SO says she wasnt very faithful and kept going back to her ex. It kind of stresses me out thinking he may have a daughter. But at this point she's going to be 4 and they pretty much have their own little family going and may do more harm than good to even do anything now.

Should I bring my thoughts up to my SO? Or just try to forget about the whole ordeal and move on? It's just all so strange.