Huge frame-rate drop in CS:GO need help

Have you checked the launch options? Right-click on CSGO in your steam library, open up Properties and under Launch Options see if there is anything new. If you have never put anything here delete all the text you see.

Also depending on your PC he may have just bumped up the graphics. If you're on a toaster try setting all the graphics options to medium. I don't know how much you know about performance, so I hope this helps.

Did you ask your friend what he changed? If you can't figure out the source of the problem then I would suggest that you try to validate the game cache or reinstall the game completely.

open console and write: fps_max "300"
enable multicore rendering in options

Did he lock framerate to 5?
Try fps_max 101 in dev console ?

edit :Ah bugger, got beaten to it :P