Well, I'm not a lawyer. (So take this FWIW.) But I do know the concept of "adverse possession" pretty well, and I'm wondering if your friend's lawyer does? Has he handled similar cases before?
I know it seems perverse, but if the neighbor has NOT paid taxes on your friend's property, that fact would actually bolster the neighbor's legal claim of "adverse" and "hostile" taking of someone else's property.
You see, the neighbor has to KNOW that he's using someone else's property, to make it "adverse" under the law!
If the neighbor had paid taxes on it, then it would look like he was merely making a mistake -- and he wouldn't have an "adverse" claim.
Amyway, I know that sounds convoluted, but that's the way the legal doctrine works. Maybe your friend should get a second opinion from a different lawyer.
The law on this differs from state to state. In DC, in order to prevail on adverse possession against the titled owner of unimproved land you must either have paid the taxes for 15 years or have enclosed the property for 15 years, in addition to the hostile open continuous notorious stuff.