TwitchSIE Start by reporting the player to the server admins. Many servers use iw4m to report a user just write !report username reason, (like !report DoktorSAS bad guy). Probably admin will need evidence to act so record the game from your prospective or/and if you can from his prospective by joining spectate team.
Remember it's not easy to find cheaters on plutonium, so it's possible that maybe other variables made you see something slightly distorted. (Like server ping, your connection, lag spike, frame drop etc)
