Where do I look for electrical problem on Yamaha Motorcycle?

If they did that, then they may have also wrecked the alternator and anything electronic (gauge head and ignition control module?) in the bike. It shouldn't have damaged the starter motor or the lights, though. Check & see if the lights work - try the brake light, that's easy. Headlight, too. The neutral indicator light should be OK.

If the lights work but none of the electronic stuff works, then try to avoid that friend when you have a bike problem. You'll need to trace power to everything electronic - anything with a circuit board inside - and verify whether that particular circuit board works at all, replacing it if it doesn't.

The alternator itself (the windings) should be fine, but the voltage regulator (probably inside the alternator but possibly a separate module) will likely be wrecked.

Nearly all electronic devices are sensitive to battery polarity, and can be destroyed if that polarity is reversed.