I agree that direct arm work helps really save my elbows. However, I pretty much agree with kursk on this point: money exercises for overall growth. You allow a newb to put some curls in his program and 90% of the time after a couple months have passed his program has morphed into "chest and curl" specialization day 4 times a week. I'm overstating things of course, but for someone as underweight as he is, there's just no point on isolation exercises when you can be doing bigass compounds. See: Starting Strength :).
Here I agree with you atg. Frequency is king when you are looking to add mass fast, especially in someone so underweight. Besides which, a beginner will almost NEVER tax himself hard enough to be incapable of doing 3x a week full body workouts or 2x a week leg days (in an upper/lower split). Once his body gets over the first 2-3 week shock period of "shit-I'm-moving-around-a lot- more-than-I'm-used-to" DOMS, newbs will be able to handle it.
There's a reason whole-body workouts are popular--they work! and they work well when applied properly. The frequency is what puts them into the scene (again, see: Starting Strength).
The reason I said 5 days a week for a successful push/pull/legs scheme is because the frequency is increased along with the volume. 4 days a week only allows you to hit 1 portion twice in a week. Average is what, 6 days rest? Kursk, you yourself recommended upper/lower like me--reason is the frequency is up. You hit every portion of the body twice a week instead of just one part. That's why I said it could work well if he's going to do 5 days lifting: then you get to repeat 2 of the 3 portions of the body in any given week, so the frequency is significantly bumped up. Everything goes back to frequency, provided you eat enough.
A newb doesn't need the extra recovery time because he hasn't learned how to coordinate his body or recruit the high-threshold motor units efficiently yet. Therefore, assuming his food intake is up to par, more frequency is better (within reason of course!) An advanced guy, yeah there's a good argument for increased recovery time with him.