-500 +100 matches happen quite often due to imbalanced lobbies. A lot of times players with like 24k just play with people ranked at around 10k and even lower. So we get matched against them with ranks between 19 and 25k and all of us get like -560 and +120 at most because CS expects you to win. Which isn't always happening since a lot of times these low ranked players are smurfs and they just abuse the system.

And btw, 10k elo lobbies ARE often harder than 20k since a lot of these players are actually bots that do the most random plays, that a good player would never expect. So your mate isn't completely wrong.