Kid Put Toothpaste & Water in My Brother’s R15 V4 Exhaust – Now It’s Smoking & Sounds Weird

So, a stupid kid decided to put toothpaste in the exhaust of my brother’s R15 V4 and followed it up with a good amount of water. Now, the bike is making a weird noise, and black smoke is coming out of the exhaust. I’m not sure how bad the damage is or what to do next. Should I let it run and hope it clears up, or do I need to take off the exhaust and clean it properly? Any advice would be appreciated before I take it to a mechanic.