That's the best solution, but he will have to have the retro rear retainer.
I would carefully mark the handguards and use a dremel. The set I have with my retro retainer were hacked with a saw and they are not necessarily straight. I'm going to probably have to buy another pair and redo them.
Those are carbine length cut down handguards that you guys have pictured, not midlength.
If you want true midlength gas system triangle handguards, contact Jhud over on the ar15.com retro forum, he cuts down and modifies full length sets to work on the midlength barrels- they look great, you can't even tell they are modified they look "factory".
For the shorter carbine length, the "easy" way is to find the set made by Tony's Custom which just need the front round handguard cap replaced with the triangle cap, and will work with the stock rear handguard retainer delta ring.
If you want to go more histotically correct, you can get the triangle rear handguard retainer cap (I thinik Model 1 sales may sell it these days) for about $25, and install it and a front triangle cap and carefully cut down a set of full length triangle handguards.