Why don't you just grab a new envelope - stick her's in there and put a sticky on it that simply says "No thanks" or something like that -- and then mail it back to her.
And I've returned mail that was opened. Sometimes it was cuz I opened it without looking, only to discover it wasn't mine. A couple times the letter was just open already (poorly closed to begin with) and they were always taken by the mailman with no problems on my end.