It depends on the circumstances.
If EVERYONE is disembarking, claiming bags and continuing by coach, then fine, if one pax decides to make their own way.
If the aircraft is staying put, waiting for a chance to fly on, fully loaded, then no, a single pax wanting to get off would not be allowed to.
Eg: TLV-LHR diverted to LTN, while waiting for fog to clear at LHR. The aircraft remained fully loaded. Many pax wanted to offload themselves at LTN because it is closer to their homes but they were not allowed to for reasons of security and the fact that offloading their bags would delay everyone else.