You forgot to teach this 28 year old to NEVER through grease down the plumbing AT ANY TIME or AT ANY PLACE including the HOUSE! Bacon grease hardens as it cools and will clog house plumbing, too!!!
Then have the 28 year old help you take all the plumbing apart and run hot water through it. They are probably all plastic fittings that screw apart. Keep plenty of rags and buckets, because cleanup is gonna be a pain.
The worse part of this is the tank. The grease will probably not run out the tank because any hot water that makes it in there will cool down and the bacon grease will coagulate and stick to the sides especially if the water was allowed to collect and sit in the gray tank. You'll need to get the water real hot and run it in there to get all the grease out, somehow! The good news that a trailer will be a lot easier to clean than a stickhouse!
If the 28 year old does all this, I'll guarantee, they'll not only never do it again, but they'll pass on the word.