I’ve liked his mom from the start. She had every right to express her frustration when Mary told Brandan she didn’t want him visiting women in his family. Only to find out later Mary was unplugging her internet and hanging with guy friends. His mom was reasonable from the beginning, and now since Mary isn’t acting like a possessive a-hole she has an opportunity to connect with her.
Remember up until that point the mom’s experience of Mary has been rude, controlling, and she had forbidden Brandan from visiting his mom and sister. So, the mom didn’t have a good impression of Mary and neither would I. Despite that experience she still tried to warm up to her. Maybe because she has a grandchild on the way, and is seeing fragile Mary is she put any issues she had with her aside.