If they received a permanent ban, they definitely had a warning, more than one in-fact assuming their behavior wasn't so incredibly toxic that it warranted an instant permanent ban. It is possible to skip to a Two Week or Permanent suspension based on the severity of the behavior in the game. There are different tiers of punishments, and they generally follow this path:
- First Offense: 10 Game Chat Restriction
- Second Offense: 25 Game Chat Restriction
- Third Offense: Two Week Suspension
- Fourth Offense: Permanent Suspension
If they had a prior two week suspension, any further violations that warrant punishment will result in a permanent suspension, no matter how minor the violation is. Sounds to me like they did more than simply asking a teammate to report someone though.
Maybe you could ask them to make a post on the boards themselves and include their chat logs? We could help get to the bottom of this and see if it was really warranted or a mistake.