The police performed an "open air sniff". Federal and Pennsylvania law differ on this. Under Federal law, this is not considered a search and can be done on any vehicle- usually to get cause for a more invasive search. (Illinois v. Caballes, 543 U. S. 405 (2005).)
Under Pennsylvania law, an open air sniff requires reasonable suspicion. This is lower than probable cause- all it requires is that a reasonable person could suspect from the facts that a crime may have or could be committed. Being overly nervous during a routine traffic stop driving someone else's car could potentially be considered reasonable suspicion- as I only have your second-hand account, I won't speculate further.
If there was no reasonable suspicion, any evidence found by that search, or evidence found by a search justified by it would be suppressed.
Fourth Ammendment rights also apply here. The police cannot detain someone solely for the purpose of waiting for a sniffer dog. They have ways and techniques to waste time for this purpose, but if the stop was concluded before the dog arrived, your husband would be free to leave.
Again, I have only your second-hand account so I won't speculate on whether this happened. He should speak to his lawyer about specifics of the case.