thief using backstab as threat

[Foril]

Hi

here is the scenario

player A is in combat, he has 9 levels of power, his opponent is a monster level 8 so he should win, the monster is worth 2 treasures

players B and C don't have any cards to alter the fight

but player B is a thief and says: "give me one of the two treasures at random or I will backstab you"

player A doesn't have any cards to help him so he accept the deal

player A takes the first treasure card face down and gives it to player B and keep the second treasure card face down.

is that played correctly ? I think that it's not but I need backup

We also had the same situation in another game but that time it was not a thief but a player who had a potion in play and was threatening to alter fights with it.

thanks