Go and find the fuse box.
Check EVERY fuse one by one.
Do not take every fuse out at once, just one by one, take one out, check it then replace it.
Look carefully in the hole where the radio was, is there any lose wires,or plugs,or connectors ?..Has he dislodged a wire or a cable.
Where the indicator switch is in the surround, can you get to the indicator switch plug that fits into the wiring loom ?...If you can try to plug and un-plug this.
Has the car a tow bar ?..Was an extra heavy duty flasher unit fitted ?..was it fused ?..check the fuse if it was.
Was there a Live feed wire directly to the stereo ?..was it fused ?...have you checked the fuse ?..Was the Radio feed hooked into the live to the indicator switch ?.
Try..try..then try.