Considering you asked this in Academia, and Academia is all about objectivity, integrity, trustworthiness and reputation: report them.
However, considering you used the phrase
it's just bothering me
I would like to give an answer as if you had asked in Interpersonal Skills:
I would talk to the one you consider a friend (or both of them). Tell them how you feel about this topic and make the moral implications clear. Explain to your friend how this is beneficial to them money-wise, but hurting their reputation and their relationship e.g. with you. Ask them to stop. If you already had this discussion or it goes badly, you should probably reconsider the term 'friend' for your relationship. Doing nothing or pretending this is ok will likely hurt your relationship anyways, at least unconsciously.
Depending on your relationship with your friend you can also consider informing them about being on the brink to reporting the actual cheater (or both of them) and you may give an ultimatum.
If you are friends, this will likely resolve the issue without further escalation. Even if not, you'll better understand the relationship you have with them, and this will likely make it easier to report them.
However, keep in mind that you cannot report them anonymously afterwards. We cannot estimate the chance of success or their potential level of aggressiveness without knowing them personally.