Even just 5 seconds of teaming is enough for a punishment.
I myself say against hackers (not skilled players) should be fine but once he's gone, you must disband the team and resume normal play. I would justify as preventing a hacker from griefing your game. By the time the game is done, he'll have griefed the game or get banned much later. Staff is too slow to catch those hackers that come and go -- they effectively avoid the ban.
Cynamooo is correct, but in the past the last 2 players in a game got false banned for teaming.