I can't see any reason why talking with him would shut down your Zumo. Presumably you pair the SMH10 with your Zumo AND with your friend's SMH10 ? So the Zumo is operating two BT channels. I sthis something you can reproduce by yourself - eg connecting the SMH10 to something else. Try breaking that BT connection every now and then, so that your Zumo has to reconnect - see if that reproduces the problem.
My XT would occasionally drop its connection to a BT module that connected it to the wired autocom intercom. The Zumo was also BT'd to my phone and and to a Garmin Drive app for obtaining data on my phone. After a few dropouts, it would stop trying, and any attempt to get it to reconnect BT would fail. It needed me to do a rain dance with the connected BT devices . It didn't exactly run away and do a complete restart - but neither did it work properly.
Problems I had was music playing very slowly, Satnav instruction sounding as if coming in on medium wave on badly tuned radio, and other weird behaviour (random key presses, over sensitive screen). As if the satnav had got itself into some sort of infinite loop which was using nearly all of it's processing power.
The solution seems to have been to move the BT module away from the ECM - which has stopped it from dropping the connection in the first place. Is there something that is happening on your friends bike that is causing the connection to drop to the SMH10, and that his having an effect on the BT to the Zumo ?
