We too sign a statement before each test that mentions about not cheating and or tolerating it.
Assuming you have some honor code to the same affect, how does your conscious allow you to get away tolerating that behavior? No one likes to be the snitch.
Let me put it in more personal scenarios to which you might be able to relate. Would you trust any of them with your family members? If you all graduate simultaneoulsy, do you want them competing with you for the same jobs? How would you feel if you end up working for any of them? If they'd cheat there, where else might they cheat?
A part time instructor at my old university caught a 53(?) year old student cheating--white socks, dark brown sandals, orange "stickees" beneath her soles so I was told. Instead of pleading I "...was stupid, mentally deranged..." or anything else along those lines, she stated something to the effect "...I don't really care about this test...I just want to get my degree so I can make more money." The instructor said he probably would have given her another chance until she offered her rationale. He decided he wouldn't facilitate anything putting her in a position to provide care to any of his family members--failed her, told university he'd refuse her as a student, and wanted her kicked out of college.
So you don't want to be snitch, understandable. Maybe the main problem is that you don't want to be highlighted as a snitch. Honor and integrety are core values to me. I gave the instructor a heads-up on rumors I'd heard. He narrowed down which classes he suspected it could be, after getting over the shock that someone would actually cheat. He then started asking students whom he felt were more honest than others "...have you heard anyone or seen any cheating..." etc. When a couple refused to answer, he informed them if he caught a cheater and could confirm in his own mind that others knew but said nothing, those in the know would also be "toast". The "rats" squealed, he targeted in on one person, caught her "dead to rights", and said nothing about what tipped him off.
Now, not all instructors would do that. Some might make you "stand up in the light of day" and proclaim all you know. Still, you could leave a typed-written anonymous note or send an e-mail from an address that doesn't reflect your name. You have options...and hopefully a conscious.