People like Mary always have to be put in their place...

...or they take, take, take. Sounds harsh but that's all Gerri and Tom did and had every right to do. They accepted this woman into their lives and she abused their kindness: drunken rants, moving in on their son, unforgiveable rudeness to his girlfriend. How were they supposed to react? Anything less than distancing themselves from her would have made them mugs.
Gerri is a compassionate and generous woman but as she says at the start of the film, we all have to make choices, and there comes a point when you lose sympathy for those who have made the wrong choices and continue making them and expect you to deal with the fallout.
Having said that, though, when the camera pans round to Mary at the end, slapped down to size, now fully alive to her ousider status in this cosy family, it is truly heartbreaking, but sometimes if someone who you feel cares for you shows you how pathetic you are, sometimes it can jumpstart you into taking control of your life.