Welcome back. Slandering the Buddha and Dhamma is a very unwholesome act. As I said, there is no specific mention of what would exactly happen to the person. In fact it is impossible to pigeon-hole (state exactly) which act would create which reaction because it is not the only action that one performs throughout one’s life. There are the good actions together with the bad ones. All intermingle; like the sand in the hour-glass. But all will pass through and "graded". To insist that every specific action will definitely cause a specific reaction is rather naive. Kamma doesn't work that way. Don't forget that we don't have to wait for the next life to experience the good and bad effects of our actions. They are experienced here and now.
To make a "calculated" guess to answer your question, I would say that landed into any hell realm is bad enough, not necessarily Avicci Hell. So let's say the person will most likely have a ticket to the hell realm! But be careful, evil retribution may occur in this life, here and now!
“And if you repent do you still have to go to hell?”
And if you slap someone, can you take away the pain from him, saying that you have repented? Kamma is amoral. A willful action will result in a reaction. The only consolation will be you will not repeat in future.
Smile from justinchoo :-)