Hmm, unusal one this....
As he already has a FAA PPL he will not be able to add his JAA IR onto it as a "based on" IR - holding a "full" FAA certificate prohibits you from holding a "based on" ticket.
So long as your friend meets the requirements for the issuance of FAA ATP (see 61.151) he would need to complete the FAA ATP knowledge test, and complete the flight test..........It is doubtful he would be able to do this in the UK, though I stand to be corrected.
But he may need to complete a BFR first in the SE aircraft, as the FAA ATP flight test is carried out in the class of aircraft for which the rating is sort. As he wants to fly a TB20 he would need to convert the ATPL to the SE ATP
May be simpler to do the BFR, using some of this time towards the 3 hrs and instructor endorsement, then just take the IR flight test (and ground exam of course)