What I reckon our budding PPL should do is revalidate in JAA land and get a SEP rating. I would suspect he will be given a lot of leeway by the examiner and it will not be much different from the checkout he will have to do if he wanted to rent in the States. So good practice. I would also guess that the biggest problem would be getting all the CAA documentation in place before he heads out to the US.
Now, what I don't know the answer to is what happens if the details on his new ATP JAA certificate doesn't match the details on his existing FAA 'on the basis of' certificate. What I'd do is call three FSDOs and ask the same question to all three. If you get the same answer then I'd be reasonably confident that was the real answer. Don't tell them you are doing this of course. I guess emailing OKC might also be in order.