This is a better question for PA attorneys since that is where the whole process to attempt a transfer would have to go. It MIGHT be possible, but I would suggest having him simply file a Request For Early Termination of Probation if that is offered in PA. Then he wouldnt have to worry about the transfer and dealing with a new probation dept. Good luck!
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Are you certain that terms of probation prohibit travel? You've posted your question in the DUI area of AVVO. In California, unless the DUI was an injury accident or at least a fourth offense, the case should have been resolved as a misdemeanor, which in most instances only involves informal or summary probation.
A formal transfer of probation is a difficult task and is time consuming. Typically the request is made with probation locally then has to be cleared and accepted by the receiving state.
It certainly seems possible, but you'll need to talk to a PA attorney to be sure. If he had an attorney on the case, I suggest giving them a call. If he has completed whatever obligations he had, it will certainly increase his chances. Good luck.