One could guestimate the following based on known factors.
200mil / 50K == 4000 BTC wagered (roughly)

Since we know that 25x is about the MAX one could wager at the point where you lose EVERYTHING (based on 5% house edge),  we'll use 25x to guestimate the deposit (or deposit total in the case of multiple deposits).

4000 / 25x == 160 BTC.

So,  160 is about the SMALLEST we could guestimate his deposit was and we could also assume that they lost it ALL since 25x is just to high (otherwise 25x wouldnt be FBC's "bonus account" bet against you).

If you go with a CONSERVATIVE guess in a scenerio where they could have actually WON (as the owner insinuated),  we would use something reasonable like 5x

4000 / 5x == 800BTC

Now check the known FBC addresses and see if you can find deposits that are around these amounts..  If you cant find them,  then you'd have to question if this was even a real account at all.  If you do find them,  well,  then you now have the exact amounts

So my guess would be between 160 and 800.