I didn't vote in the poll because I think that's taking the wrong approach. I think your friend needs to make a list of what features are important to him and then look at the guns that match all or most of those features, or are cost effective if changes need to be made.

Upper receiver - Flat top (optics) or fixed carry handle (A1 or A2).
Barrel length.
Barrel twist.
Barrel profile.
Barrel material or lining.
Barrel/forend configuration (standard handguards or free float tube).
Gas block/front sight tower.
Buttstock, primarily buffer tube style (carbine or rifle).
Furniture (CAR, M4, A2 rifle, Magpul, etc.)

Once your friend has determined what specs he wants for the above features, or whether any have more than one acceptable spec, then he can make a better choice from the available M&P 15 guns, or even other makes.