I had a very similar experience. We invited a player to join our FC and he was actually a pretty cool person and stuck with us for awhile. We even helped him clear a few primals and built some trust with that player. However, one day we gave him an upgrade to access privileges and ranked him up which was a huge mistake. We overlooked a loophole in our rank system and he ranked himself as one of our FC officers and proceeded to kick everyone and raided our FC chest. He also left a message trying to frame me on the message board but everything was obviously on the activity list.

So we reported it and the player got punished according to our FC leader. Problem was we lost everything including our members. We never gave up though, a few other FCs in the community learned of what happened and helped us regrow. Our kicked members found their way back and surprisingly became 10 times more active. 1 year later we are still standing strong with the same members, several more joined, and working on Alexander Savage. Ever since then we've been really strict with our access privileges.