colej If there are back taxes on it you better pay them.
I would do this. Even if no proof of ownership can be given for your wife or the tenant, you can legally own this in a period of time. If he cannot show ownership, he's screwed, unless he pays them up 1st. If the grandmother is deceased, and the property is in her name, then whoever gets there 1st will get it.
Unless there is something in a will stating ownership of said property.....did you think to look at that?