Austin - First off, welcome to the forums!
There's nothing worse than a hack who lowers a car incorrectly, or without doing their homework first. Sometimes people will sacrifice safety and handling just for "looks", very stupid in my eyes.
So the first thing you need to do is determine how the car was lowered. Did they cut the stock springs or did they install aftermarket lowering springs? Regardless of how they did it you're going to have to replace them with stock units if you want to get the ride height back. Unless of course they installed coil overs... if that's the case you may be able to adjust them and raise the ride height.
Lots of guys who install aftermarket springs on the Mustangs will then put the stock springs up on eBay for pretty cheap. Just keep in mind that if you are going to replace the springs they installed or modified you need to ensure you get the correct ones back in there. The springs in a Mustang 5.0 are not the same as the springs in the Mustang Ecoboost. This is because the cars weights are different so the springs have different "rates".
So you'll need to get the car up in the air and take a good look at what the previous owner has done before you can determine what corrective steps you'll need to take to rectify the situation. Report back once you know what you're dealing with.
2016 Ecoboost Mustang
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)