Let me answer your question by advising you and your fiance that he needs to discuss his situation in private with an experienced criminal defense attorney. Only in this manner can your fiance make the best decision whether or not to fight extradition. There are often strategic and timing considerations that affect the decision that are beyond the scope of an easy answer on a public site. So that you know, extradition is simply a request made by another state's governor to our governor to produce and return an individual to the requesting state to face criminal charges. The process is fairly routine. Assuming the actual paperwork is in order, (and it sometimes is not!) the primary question for a judge in Pennsylvania is to determine if if your fiance is correctly identified as the person that NJ is seeking. The judge here in PA will not consider the merits of your fiance's guilt or innocence of the NJ charges. Good luck!
Attorney Mishin has as usual provided a thorough and accurate answer to the extradition part of your inquiry. Your fiance needs a good attorney in NJ to defend against that state's charges. The merits or strength of NJ's case against him is NOT a consideration in extradition proceedings. If money is no object then by all means secure PA counsel to advise him on the extradition and one in NJ. If funds are limited focus on defending the NJ charges.
Judges do not generally inquire into the merits of a case when considering an extradition request. The only issues are (1) whether there have actually been charges filed, and (2) whether there is sufficient reason to believe that person for whom extradition is being sought is actually the person against whom said charges have been filed.
If the answer to both of those questions is "Yes," then off he goes. Doesn't matter if the charges themselves could wind up getting dismissed in the jurisdiction seeking extradition. That's something that would need to be dealt with there.
As such, fighting the extradition may not be a terribly good use of your resources. You will probably be best served by devoting those resources to defending the charges themselves in New Jersey by hiring an attorney there.
This answer does not constitute legal advice and does not establish an attorney-client relationship.