Конечно правомерно, в
правилах версии 1.5 даже есть специальная врезка по этому поводу:
Цитата:
Interfering With Combat
You can interfere with others’ combats in several ways:
Use a one-shot item. You could help another player by
casting a potion against his foe. Of course, you can “accidentally”
hit your friend with the potion, and it will count
against him.
Play a card to modify a monster. These cards (usually)
make a monster stronger . . . and give it more treasure. You
can play these either during your own combats or during
someone else’s combat.
Play a Wandering Monster along with a monster from
your hand to join any combat.
Backstab a player in combat, if you’re a thief.
Curse them, if you have a Curse card.
А со стола обратно в руку никогда ничего идти не может (стр. 1 правил): "Cards in play may not be returned to your hand – they must
be discarded or traded if you want to get rid of them."