I would have no issue with them bringing him in to camp.
It has nothing to do with record as starter - teams have records, not players. He would be told that he is expected to be a team player and support stater. He would be told he needs to do whatever the Bills need to do, even run scout team.
I would expect him to want to compete but he would have to understand that Fritz is starter unless evidence is overwhelming in favor of him and while offense may have a few plays inserted for him it will not be retooled for him during this off-season. He would need to compete to even get a roster spot.
if the Bills do bring them in they have to either have a contract with him which is incentive-based or that they can afford if another backup QB position is cut,