She picked him, knowing that he'll not be a threat to her position, and it makes her look good. He may be even a kind of "shill" for her.
In her absence, he has a legitimate reason to be the "acting" manager. You consider him your peer is not realistic.
He can certainly see that you don't want to subordinate to his "leadership." Should you?
We should be more aware of what we are thinking, because when we are not, others will be.