A friend was putting this in the attic space over the garage and caused a seam to split in the ceiling. There isn't a hole just a two foot length where the popcorn ceiling is split down the seam and sagging downward on one side. It is only inches away from the ceiling fan. How do I repair this?

Your firs problem is having a popcorn ceiling. No matter ho good you are, you are going to see the patch. You are going to have to treat it like latex paint. Find the edges of the bubble and cut it out with a utility knife. Make sure not to cut through the drywall.
On the otherhand, if it is the drywall itself that is sagging, you can screw it back up. Make a couple of deadmen "2x4's nailed together like a capital "T" an inch or so longer than the height of the ceiling. Use these to put pressure on the sheetrock to hold it tight to the joists again. Screw the boards back up DO NOT NAIL THEM. Using drywall nails on a ceiling is a huge mistake. A lot of companies do because its a lot cheaper, but after a few years, you start seeing circular bubbles in your home from the wood drying out and releasing the nails.