Can see no reason why not so long as the PoA is sufficient in its scope. The chances are that it is unless it is limited in some way.
If there are no limits teh attorney can do anything the donor could have done.
You also need to look at the wording of the power and the donor’s situation now and when it was drafted. I think that if she has become unable to act during the intervening period the PoA should have been drafted as an enduring power of attorney, otherwise it will have no effect.
There may be questions about the way in which the attorney is acting regarding, for example, sufficiency of rent but I would not even go down this line unless you are very very confident of your ground.
Although there appears to be a question relating to commerciality on the face it I think the correct approach would be to consider what granny would do if there was no rent coming in and the attorney took no action. If you are satisfied that she would revoke the power or take action iin some other way I do not think that there is any reason to dispute it.