Yup sounds like a wiring issue.. Does your friend's guitar work through it at all?

My sub-woofer died on my home theatre system the other day.. luckily my dad has a bit of knowledge of electronics and stuff so he had a look and it was simply the wiring where the push-button switch is. Somehow it had become fused and the switch no longer did anything. So now it is always on, when connected to the power socket, as he fixed it by bypassing the switch completely. Hopefully the problem with your amp is as easily fixed.

Definitely take it to a repair person.. they will find out what the problem is and fix it.