Jon is absolutely correct. Your mortgage likely disallows this arrangement, and the tax complications are a headache, you don't want to get involved, especially if they have shown to be late. The legal headache of evicting someone who has "equity" in the arrangement virtually ensures you'll end up in court.
My advice would be to download a sample agrement (
http://www.agreementsetc.com/rental-...t%20Sample.htm) and go from there. I own a lot of rental property and I have a good agreement in word version I can get to you if you don't want to go through the hassle, just PM me.
Lease to own arrangemnts are typically a poor idea from the leasee's perspective anyway, there is a moral hazard here for you to consider.