Originally Posted by Heidi:
I would mail the check to her old address. If she forwarded her mail, she'll get it, cash it, and you'll have a cancelled check to prove you returned it.
If she didn't forward her mail, you'll get it back. Leave it in the envelope marked "return to sender", unopened, and put it in your files. After 6 months, the check will be stale dated and you can safely assume she's not wanting her refund. 
This is excatly what I was going suggest. I think it is the best option. I
Originally Posted by MyAngels:
Even if you were to put that notation on the check, the bank doesn't have to honor it. The only way to be sure the check won't be cashed eventually is to put a stop payment on it.
I would call my bank and ask. My CU would not cash a check for my auto loan, written off my checking account from that CU, because I wrote the previous year date. (this happened a few years, at the beginning of the year.)
So I would check to see what they say.
Originally Posted by Thriftylady:
I just don't see why the OP should have to wait months or years and then offer a refund. If the parent wants it, they should make accommodations to get it in a reasonable amount of time. I think 30-60 days is more than reasonable.
I agree