"When you face a challenge together with the other PCs, don’t roll dice separately. Instead, you choose who among you is best suited to take on this challenge. The others may help them (see below) if it’s relevant to the situation. If the roll fails, it counts as a failure for all of you—you are not allowed to try one time each. This rule does not apply in combat, where each PC is free to attack any enemy they like."
I read this to mean that a group failure does not mean you cannot attack individually. Not that you cannot help in combat.
Regardless, the rules p. 37 read "In combat, helping counts as the same type of action as the one you are supporting (fast or slow, see page 53)." Helping is explicitly permitted in combat.