I think Lydia was planning on going to the police but then Sam started banging on about what a great guy Craig is and I think it put a doubt in her mind that noone would believe her, not even Sam.
I don't think the clothes were bullet proof evidence as no doubt Craig will say they did have sex and it was consensual. He's played a blinder by pretending to be Sam's mate and it makes it even more sinister and calculating that he's done that.