Her due date doesn't add up right?

So my bf has this girl from out of town that has been saying she's pg'd from him. We got together about 9 months ago and he slept with this girl once right before he and I started our relationship. He said he hung out with her twice & slept with her once. So he says he was with her in the begning of March at a party which I remember was around the 15th of March. At first she told him her due date was around the 9th of Dec. which based on that due date there was a good chance it was his. I talked to her finally in Oct. and she confirmed that she was with him in March to me but we didnt; talk long cause she started cryin and said she couldn't deal with this at the time. Fine I let it be, well since then we hadn't heard from her. He finally called her about two weeks ago to see what the status was and she tell him that she is due the last few days of Dec. or by the 1st of Jan. So now with those dates, there's no way she got pregnant in March, she had to have gotten pg'd in April, early April. I've talked to him and told him if he's lying to me about when he slept with her because the dates dont add up and it couldn't be his. He says no that he was with her in March and that she also told me that. She also told him that he could name the baby, which is a girl. I'm not liking this one bit and I don't think he should be around until he gets a test right? or am I being the jealous gf? And if this kid does end up being his that means he slept with her after me and him were official which I will have a big problem with. I haven't held it against him cause I've been under the umption that he slept with her before me but if that's not the case we have problems. I dont know how I'm going to deal with him being around that baby until I know for sure it's his and he says he dont know what to do. She said she'd call him after her last appt. and I know she's going to call the day she goes into labor. I'm scared what he's going to do. Would it not be right for me to go with him to the hospital? And I dont want her to give it his lastname until he knows for sure. All this that I'm feeling is it not right or do I have a point?? And don't you all agree that the dates dont add up??? plz help it's getting close and I'm freakin out.