charmed by a monster

So one of my party members said that if a player is charmed by a monster and you hit them the charmed effect ends even if the effect says creature charmed by it is charmed for 24 hours is that true

It differs depending on the monster. For instance, the Autumn Eladrin has this clause:

Whenever the eladrin deals damage to the charmed creature, the creature can repeat the saving throw, ending the effect on itself on a success.

The way 5e works, the rules about what ends the charm are always going to be within the feature that causes the charm.

Agree with above. We fought an Incubus a few weeks back. Its charm can be broken by if the target suffers any harm or receives a suicidal command.

It's all in the wording of the spell causing the charm effect.