It must have been scary for someone as traditional as Mary to realize she had a bit of an oddball on her hands; I can see the woman doing a lot of worrying about what type of life he would have, especially since so much of his behavior had to have been an unknown quantity for her.
So, of course Mary is just glad he’s finally close enough to a woman to be comfortable living with her.
When you think about it, Mary’s reaction really isn’t that surprising.