What time frame are we talking about?
I (stupidly, although it worked out) signed up to do the Olympic distances/day for the month of June after being out of sport for about 25 years (and being 64 years old). I ended up losing about 12 pounds (i.e. 5.5kg).
I did the workouts at a pace that made them feel easy (and, boy, did it take a long time per day starting out), but I controlled my eating and did the exercise.
I started each day with a protein shake (160 cals/30g protein) and if I felt tired I added a regular Ensure (220 cals/9g protein). The protein is necessary to avoid the muscle loss. I ate 2 meals a day (that’s the rhythm around my house), but avoided eating the foods which I felt had too many carbs for my purposes. (My wife, fortunately/unfortunately, is a great cook.) I took more Ensure or the protein shake as needed to make sure my energy wasn’t bottomed out and I got sufficient protein for the day; the need for protein is considerably elevated when one exercises a lot (and, in my case, due to the increased intake necessary due to the increased inefficiencies of the system in processing protein as one ages.) If I felt I had depleted glycogen stores I made sure I got some carbs immediately after the workout so as to replenish them so as to feel fresh for the next day. (But never excessively many carbs, as I was burning mostly fat.)
By keeping my workouts easy I made sure I was burning (a lot more) fat and not catabolizing much muscle, since I needed to be able to come back each day. I also (when I remembered) made sure I got some caffeine 15-30 minutes before the workout to start the system putting the body fat into the bloodstream so it was available as an energy source at the beginning of the workout. (I’m using info here from over 20 years ago, so this may not be current, but it works for me.)
If the hills are too difficult, have him get a trainer. I ended up getting one at the end of the month because of the numerous (human and goose) obstacles on the paths that I was taking (one of which caused me to crash), and the workouts became much easier and were available 24/7.
In addition, doing the three sports allowed me to do much more exercise than I could have done with only 1 or 2. During the time when I was averaging over 100 Slowtwitch aerobic “points” per day I put in a week with over 42 hours of exercise. There is NO WAY that I could have done this focusing on only one sport; in fact my “points” were almost equal in the 3 main disciplines. Obviously, your friend should be able to maintain higher work intensity at the same level of perceived exertion than I did.
Finally, if it’s that important, why is your friend only limited to 14-15 hrs/week? I found by going more “gently” I needed less sleep, and if he can get away with 1 hour less sleep and use it for training, he can up his training time by 50%.
I hope this helps, and contact me if you have more questions.
battle. I also found that glycogen depleting workouts (even loooong z1/z2 efforts) drive appetite. So, even base-extending types of workouts seemed to end up being either calorie neutral or positive. Otherwise, again, I found myself descending into a losing battle for recovery. Anything that required me to bring nutrition was counter-productive. For me that limited things to about 90-120 minutes of training max per day…and, I stayed away from any threshold / Sweet-Spot type stuff.