Anyone else think it is really weird that when the scripted is flipped -the fl's best friend is dating a guy that is in a fake partnership so that the girls sick mom is not worried anymore.- that the fl's best friend dose not believe the guy's story even tho it is basically the fl's plot? I find it very odd

It's more like he hid it from her from the get go. Had he told her about the ruse in the first place, she wouldn't have been so angry. He didn't trust her enough to tell her what's going on and when the ex-girlfriend introduced herself as the girlfriend, he could have corrected her immediately instead of keeping his mouth shut.