Yes, it is possible that there was interference from an outside source, especially if the wireless system is 10 years or older. Many of the older systems worked on VHF frequencies, which is the same frequencies that fast food restaurants, wireless intercom systems and some hand held walkie talkies used. Some of those systems had a permanent keyed mode so the person on the other end could listen constantly, but could not respond.

The newer systems are usually on UHF channels and don't usually have those problems. I would recommend that he connect the system up at home, and be sure to include all other instruments that are usually connected to his PA, and see if the problem recurs. If it does, the next step is to disconnect the individual components one at a time and see if the problem goes away. It not, it's time for a new wireless system.

Hope this helps,

Gary