Can my son's guardian pay for her legal fees through his estate?

Guardians have to answer to the state. They cannot spend wards money for themselves.

If she has to pay the legal fees because of work she is doing on your son's behalf, then yes. That would include fees she has to pay because you are opposing her guardianship, just in case that's why you ask.

If the fees arise from something completely different that has absolutely nothing to do with your son, at all, then no.