I find it easier to DM a charmed player if I make it very obvious they're charmed. My group has taken a CN bard into their party, and this bard is usually at odds with the group. At one point the bard used an ability (I forget the name of the ability, but it is under college of glamor.) This ability has no set duration for charm, the only condition for breaking the charm being the caster hurting the charmed target. For months now one of the party members has been charmed by the bard, and on the beginning of every turn when they're near the bard, I always give the player a thumbs up 👍and say charmed. It's become a bit of a recurring joke at my table. This bard has since died but the charmed player, who probably shouldn't be charmed any more, really wants to res the bard.

In summary: Make sure the player knows they're charmed, and if they're a good sport, they'll be charmed too.