With one of my teams, I have essentially the same team as I did a season ago, and have gone from a 101-win team to maybe a 65-70 win team. I haven't tanked it. The power update did a number on my team, and a lot of my pitchers are having off-seasons.
I think you'd have to provide additional information to justify your reasoning of why you believe it's a tanker, before any reasonable conclusion could be made. It could just be the instance of where a team is just simply having a bad year.
EDIT: I see you've provided some additional information. However, all I would do is ask him for an explanation, before accusing him of being a "tanker."