I had a mastectomy in March 2010 with immediate reconstruction (expander placed)
Your mom will wake up with a surgical bra on that will provide support to the chest area.
Of course she will be sore - so any movement, I clutched a pillow to my chest and it felt better.
She should wear loose comfortable clothes - button tops are best because range of motion will be limited. Be sure someone is with her for the first day or 2 because she will need help to get up out of bed or a chair even.
It's difficult to remember everything but please keep in mind, your support for her is the most important thing. In a week or so, she will feel a bit better. I was off pain med after 2 days - everyone is different though - so just be sure she takes it easy and if she is feeling good - still take it easy. Do any exercises the doctor recommends to help with range of motion.
And we'll say a prayer for her recovery.
do you have any specific concerns?