The in-game mechanics are likely easier to handle than D&D 5ed. If there is a player that struggles with mechanics, offer them pregen/heavy help out of session to set up the character; but in actual play you can distil the mechanics quite a bit. For instance having a personalised cheat sheet with only the most likely maneuvers and actions for when they have their turn.
I am trying out that to introduce my brothers. I have prepared such cheat sheets, with a basic version (2 most likely maneuvers and actions, no heroic resources), and a more advanced version (~4 most likely maneuver/main actions including heroic resources) - alongside a seperate "card" with what they need to keep track of outside their turn (triggers/heroic resource income, 3 things each). Each of them personalised.
If there are no stigma connected to it (the player know who they are) playing a retainer might also be an option?
Out of combat should be very similar to 5ed in complexity.
