Stolen MacBook running Windows via Boot Camp My friend's MacBook was stolen. He was mostly using Windows with Boot Camp. macOS was not used at all.

How likely is it, that the thief may be able to reset Windows password?

Is it possible to transfer the hard disk to another computer and access it through Windows files.

And of-course, I don't want specific information about resetting password for accessing files etc. Only a confirmation what might be the worst situation.
You didn’t provide enough detail to adequately address the actual problem (I.e MacBook model, version of Win10, was it unlocked/unencrypted, etc ),  so we must look at this from the worst case scenario:

Once a device is physically in the possession of nefarious parties, you must assume the entire system is compromised. 
Guessing what the risk is at this point is an exercise in wishful thinking.  

This is where you advise your friend to start securing accounts (change passwords, add two factor authentication, etc.)