They can't deny claims based on anything in your previous history. They can't use your history of medical conditions, history of implants, history of drug use, history of pregnancy, history of employment, history of not wearing seat belts, history of anything.

They can say, "We won't pay for this MRI" or "We won't pay for this drug", but that would be true of everyone else on the same plan, regardless of whether they had an implant.

More typically, they say "We only pay for a certain number of MRIs per year" or "You have to get a cheap Xray instead of an expensive MRI" or "You have to try the cheap drug, if that doesn't work then you can try the expensive one" or "We need to be notified 30 days prior to getting the drug, otherwise we won't pay" or "You can only get the MRI at this other location, otherwise we won't pay".

None of this has to do with your medical history, though. They are simply annoying hoops that everyone has to jump through. And they can never, ever, say "We will pay for X only if you remove the implant, otherwise we won't pay for X". If they are paying for X for anyone else on her plan, then they have to pay for hers too.