If that's true and his coins were stolen, what's the course of action? I don't know how one can prove that they had their BTC stolen from their account unless you have IP logs.
Generally there isn't any action. You had your coins stolen, and now they're gone.
Stunna can presumably tell OP the IP address that withdrew his coins. OP won't be able to prove that it wasn't him doing it. The scammer will have used an untraceable VPN, Tor, or some such.
The best that happens is that PrimeDice implements 2FA-on-withdraw to stop this happening to others in the future (this would have prevented last night's "exploit" attack too), and OP steps up his own security to stop this happening to himself in the future.