Should I tell somebody if I know that another student is cheating?

Well, it's definitely not the right way to go. However, maybe 你 should try talking to her about this first before going to the teacher. If she doesn't want to hear it, 或者 if she just completely ignores you, then 你 could tell the teacher. However, the teacher shouldn't only talk to the student; she should also talk to the parents. I understand that parents want their children to do well in school, but how are they going to learn if they don't do things themselves? That's not very good parenting (although there could be worse things)...

Furthermore, what's going to happen if her parents are too busy to help her? Her grades will tumble; it won't have any supports. 你 need to let her know this too. Your parents can't always be there for you, even though they want to.

She also needs to learn to work hard, set some goals, and build some character. I really hope that she doesn't have any younger siblings -- that's not setting a great example. Sorry for the rants here, but things like this just annoy me. No one seems to want to learn.

Approach her with a calm composure, and speak reasonably. 你 don't want to be angry with her from the start, for 你 won't go very far in your little discussion.

She's not gaining any knowledge letting her parents do the work, tell the teacher. In the end, that girl will be thanking 你 later on in the future for doing that.

Sometimes 你 should let things go, but later in life she's gonna face the consequences on how wrong she is. If that dosent work 你 should tell the teacher.

say nothing, as it could be a trap. many say things yet change a little of it each time, so 嘿 can find out who is the mole. best 你 let her go, surprise tests do happen, then what grade will they go back to. mum can dad wont be able to help all the time, and let her fall into her own trap, your grades 你 deserve, they are hard and well deservd, hers are just temporary, life wont give her A s later, and theyre the ones that count in any future,;-]

This kind of Situation is tough, i know. But letting the teacher know right away isn't what 你 should do. I would consult with her first. Make sure she is telling the truth. Because i know many kids who are very knowledable and don't want to be called nerds so they act like someone else is doing all the work.
If she says thats true. 你 let a teacher know in a way that will make it sound Important and not Tattletale. Maybe saying something like "Can i talk to 你 about something important" and if they say yes then 你 carry on like this "I was talking too (insert name here) the other day, and she brought up that her homework is done 由 her parents." maybe 你 could ask the teacher to make a popquiz that is not for a grade that will prove that she is cheating.