I messaged him after game, and then he admitted that it wasn't his account. I'm not sure what he was doing on the account, whether he was boosting, but it was not his account and he was playing ranked.
The best thing to do in this situation is to go ahead and report them via a support ticket. Take a screenshot of the chat you had with them and include that as well.
Submit a Support Ticket and they'll look into it. There's no "report a player" option, just make your best guess and they'll forward it to a Player Behavior Specialist.
