think friends gf is a liar and don't know what to do

Ok so my buddy has this girlfriend and they have been together on and off for six years. Never once used a condom and she's never been on birth control. Well now she's pregnant. I don't think it's his and she got mad when he said he's going to want a paternity test. They just got back together December 5 and she got a dark positive and a clear blue digital that said 3+ weeks on it on December 20. I don't know about you guys but that sounds funky to me