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.
