What would you do?

My so and I took a 7 month break last year. Got back together in July. During that break we both slept with other people.

Dec 2015 a girl came out of nowhere saying she was pregnant and it was his. I know their last day together was July 3rd and her due date was April 18 2016. Based on conception dates there was almost a month difference so I pointed that out and she admitted to sleeping with someone else consistently after him, which if she had a regular cycle would make him thd father.

I told her that we would pay for a paternity test right away, she declined and said she wanted to wait until after the baby was born. It's been over a month since her due date and we still have not heard from her.

She has deleted ALL of her social media accounts, she isn't responding to emails, she doesn't live at the same place and we don't have her number.

How would you go about this? Would you just forget about it and assume that she confirmed the other guy was the father, that something happened and she didn't/doesn't have the baby or would you involve a lawyer?

I don't want this to creep up down the road and have her surprise us with this child if it is his. Would rather sort out child support and the situation before hand but we can't find the woman in order to do so. I'm freaking out a bit I hate the not knowing, my so is assuming that she figured out it wasn't his and just doesn't want anything to do with us anymore or she was lying and knew it wasn't his and was just trying to get him back (she took it very hard when he stopped seeing her. Over dosed on drugs to try and get him to come to the hospital to see her, non stopped calling him, harassing him. He finally had to change his phone number)

There was no more then 5 emails exchanged on the one day when she told us, she never tried to contact again. It's so weird!