I'd be angry if my parents opened my mail too! There are just some things that ya don't do. But, then again, if she doesn't want to risk it, she could get a P.O. Box and have her mail sent there.
It's nearly impossible to detach from our own children. I feel so deeply your plight. I just know that the ONLY thing that worked for me is when my grandson was put into protective custody with CPS in my home and taken away from my daughter. My friends would see my kid out at the club when I was up with her sick child. These kind of circumstances are tragic but somehow facilitate detachment.