[edited by: Webwork at 2:47 am (utc) on Mar. 16, 2007]
[edit reason] WebmasterWorld TOS [/edit]
The lawsuit may include a claim for past damages, which predate when the domain was 'dropped'. On that score there may not be much he can do, except offer whatever proofs he has.
If he's poor then he should apply to legal services for representation. He should also contact the local lawyer referral service and possibly the local law school, which may have a legal clinic where 3rd year students help people in his shoes.
If he's clear that he's not the owner then he may be able to file a 'third-party lawsuit' where he names the new registrant as a third-party defendant. This might cause the plaintiff's attorney to amend their pleadings to join the other party. However, if the lawsuit includes claims for other 'bad acts' that may not be the end of things.
As poor as he is he can probably afford an extended consultation. He should go well prepared, with all his paperwork, including image captures from Archive.org, copies of the WhoIs record as it now stands, any documents showing his past ownership/control (payment, etc), evidence of who is doing what now with the domain (screenshots, etc).
A letter from a lawyer might convince the current registrant to pull down whatever now exists.
Good luck.