How do I release the 3.5mm jack plug from the cable connection point.

My grandson was presented with a "brand new set" of HDJ-2000MK2 phones (Not ones used in the booth) as a thank you for doing a set on the Pioneer stand at BPM Birmingham (UK) this year. A couple of the Pioneer guys spotted him mixing on the CDJ 2000 Nexus and invited him to play on the main stage. The 3.5mm jack on the cable connection will not release when he tries to remove it from the phones. He is trying to get in touch with the Pioneer guy who gifted them to him to see if he can get them replaced at a local dealership. Any ideas on what to do.