I'd house rule it this way. A warhorse is trained for mounted combat, but most PC's aren't. Trained mounts respond to signals and an untrained PC doesn't know the necessary signals and riding skills to direct it effectively.
If the PC has a warhorse (or other mount likely to know how to take part in combat) they can either Attack or control the mount during their turn as per the rules in the PHB.
If the PC has a warhorse AND the Mounted Combatant feat, they can use a bonus action to direct the warhorse to attack once per turn (2d6+4)
Yes, it'll be quite powerful, but keep in mind that;
a) The PC burned a feat for this, letting them do something cool is just going to increase their fun, and fun is the end goal of every session.
b) Realistically, trained warhorses DID dramatically increase the combat effectiveness of trained riders; this is why mounted combat was so important a military skill for so long.
c)In the average campaign, the PC will get VERY few chances to use this feat. They will get to be the MVP of less than a handful of combats at best. It will allow your Fighter to double his damage with a bonus action once every blue moon, whereas your Rogue gets to do that every time he gets a sneak attack. Let them have it, it won't break anything.