I happen to agree with Lineys. The thing is, in itself, the pocket condom is not evidence of cheating, but it's definitely circumstantial evidence of an intention to 'be prepared' should that opportunity arise, at a guess, on the night that he had out recently with his brother. (that';s not to say of course that he hasn't cheated it's just that the unused condom does not in itself prove that.)
If all he is going to do is stonewall the OP then she is in a really hard position-
dump on the basis of circumstantial evidence of an opportunistic intention to cheat
or
give benefit of doubt. IN the DP's defence, he hasn't tried to concoct a 'oh my mates form work did it as a joke' or come up with other bull. He's done a straight denial. it might be true. However, in his soes, I'd be mortified and wanting to get to the bottom of it., THePD amazingly seems quite chill at the prospect that a condom fairy has been at his pockets.
TBH, I can understand her going for the second option and trying to forget the mystery . it's certainly the easier option right now.
however, it's out there now, isn't it. Pandora's bloody box is open. Where the very fuck did that condom come from?, she will be wondering for years (or less if he trips himself up before then.)