I can only speak to guardianships in Texas, but perhaps the Georgia answer is not too far off. Your question is a bit confusing inasmuch as you indicate that your brother-in-law is both agent under a power of attorney and legal guardian for his mother. Those two don't usually co-exist, and you may be referring to him as "guardian" simply as a description. Ordinarily, when a guardian is appointed for an incapacitated individual, any power of attorney previously in effect is terminated.
Again, Texas may treat guardians differently, but a guardian in Texas might be entitled to what is effectively a reasonable commission. This compensation is typically requested, considered and approved annually when the Court also reviews and approves the annual accounting submitted by the guardian. I wouldn't generally encourage anyone to submit that accounting or a request for compensation without the assistance of an attorney.
If, on the other hand, your brother-in-law is only actually acting as agent under a power of attorney, he has a responsibility to act in the best interests of his mother when handling her affairs. If she could pay someone for the kind of work that he is doing, then he can likely do the same. The safest bet would be for any compensation to remain reasonable given the time and/or labor involved, and you might encourage him to take a few moments to discuss the matter in greater detail with a Georgia attorney familiar with powers of attorney.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should not rely on this answer alone and nothing in these communications creates an attorney-client relationship.