Her urge to help her son is strong, she is a mother. I don't know maybe she didn't but she knows in the back of her mind that in all reality she needs to quit drinking too. In his mind what right does she have to have them come and lecture him about his abuse when she is just as bad. What is she going to do if her son starts telling them her "dirty little secret". She may have guilt or began to feel like a hypocrite.

Maybe not, maybe it was just as simple as her being afraid he would not be home, but in her position would you really have expected the son to stay there, honestly, if you were him would you have? She may very well have had time to think about it and decided that her addiction was at risk so she may have sabotaged the meeting. Maybe in her mind it wasn't in those exact terms.

People are funny and can do things that they aren't even aware of especially when it comes to addictions.