Lance: You have disadvantage when you use a lance to attack a target
within 5 feet of you. Also, a lance requires two hands to wield when
you aren’t mounted.
That's pretty straight forward, but, by "my player's" interpretation of the rules, the reason this is worded like this in the game mechanics is that a Lance unlike a Spear is designed to be used from horseback, would an Aasimar of the Protector variant which has a 30ft flying speed for 1 minute…
Radiant Soul: Starting at 3rd level, you can unleash the divine energy
as an action. When you do so, your eyes glimmer and two luminous,
ghost-like wings emerge from your back and you gain a flying speed of
30 feet. The transformation lasts for 1 min, and once on each of your
turns you can deal extra radiant damage equal to your level, whenever
you damage a creature with an attack or spell. You can cancel Radiant
Soul with a bonus action, and cannot use it again until you take a
long rest.
…Could the Aasimar's "flying speed" substitute the "mounted" requirement for the lance to be used one-handed?
(I guess that the Aarakocra natural flying speed or a "Fly" spell cast on any non-flying creature could have a similar effect).
Best Answer
Flying does not equal Mounted
Being mounted is very different than flying. The Lance specifically requires you to be mounted in order to avoid the penalty for one-handed use. Mounts are covered in the PHB (page 155), and nowhere does it state that flying/swimming/walking yourself would ever qualify as a mount.
In order to be able to wield a Lance one-handed without disadvantage, you would need a legitimate mount that you are actively riding.