Help!!!

Okay I need advice. My husband just got a call from a girl saying she had his baby 3 years ago. The child looks exactly like him and we desperately want to be there for the baby, if he is his, however the GA on the ultrasound is 8 days after they had sex (it was a 1 night stand lol), could an ultrasound be off by 8 days!? After figuring out thar we want to be there for the child the mother now is denying it is his after all this trouble and worrying and doesn't want to pay $500 for the DNA test.