In order for you to claim it as a deduction (and you're entitled to) she has to claim it as income and pay taxes on it. There's nothing wrong with you sending it certified mail to the "last know address" either. Then when it's returned as undeliverable, you'll include that notice, along with the W-9's in your paper filing. You'll still get the exemption, and in time, the IRS will hunt down the provider and take care of things from that perspective.
If you don't want to do this, because the friendship means more to you, I can understand that. You just can't claim the childcare expenses then.
