Yes, I understand that, but if there's anything I can do not to have to replace it, I'd much prefer because it's so hard to get to AND it's my daughter's house, one toilet and two kids!

If there is evidence of corrosion at the fitting, a leak is a likely consequence. Better to replace it as a scheduled repair than on an emergency basis.

There is no way to avoid the inevitable! Steel connected to copper is a classic example of a galvanic corrosion cell. The copper will be reduced while the steel will continue to corrode. dielectric couplings can break the corrosion cell.

Perhaps a whole house RO system, or follow Charles' sensible suggestions.

IMPO

SR.

Unless you replace it with a dielectric fitting the only question is: When will the leak occur and how much damage will it cause? There is no magic fix for bad plumbing of this type.