I never understood supplements, most of it is just dried milk, then you add water and viola you have "whey protein".
If the person is doing an adequate amount of exercise with proper nutrition, there is fairly little reason as to why they shouldn't get bigger.
Milk does help you lose weight, so if you are drinking a lot of milk to gain weight, that might be detrimental.
My dad was 160 in HS at 5'10" (18 years old) and was declined a full scholarship for football due to being so light, he was number one in the state for "tackling" (I don't play football so I don't know the position or remember what he told me) and he tried all of the supplements, ate 6+ full meals a day, exercised like crazy, and drank as much milk as he could and never gained weight. The reason I believe and he believes he never gained weight, was because his exercise either equalled his caloric intake or it exceeded it. That and milk helps you lose weight so drinking a lot of that didn't help him any.
Personally, I never used any supplements and I gained muscle mass/weight just fine, I think it is mainly the kind of exercises your doing and the reps/weight amount. And your diet. I always ate more calories then I expended, and I gained weight (Not fat, but muscle). I was about 135-140 in 8th grade, 165-175 by the end of 9th grade. I don't know an exact body fat percentage but it was fairly low. I was eating anywhere from 3,500-5,000 calories a day and exercised for atleast 3+ hours a day. At some points 6ish hours. (Water Polo+Swim+Weight Lifting+Walking/Running)
It's fairly common knowledge that high reps at low weight build endurance, while low reps at high weights builds mass/high bursts of strength. I would just advise the people who want to gain weight to continue eating a well balanced meal, add some more carbohydrates and proteins, and then just continue to exercise. Don't forget calcium as your bones will need it, as they will "break down" and "rebuild" just like muscles do when you are lifting heavy weights.
I don't know all the scientific terms for it but the bones are porous at a microscopic level, and with weight lifting they become less so, and eventually they will become completely solid as those "holes" are either filled up or broken down and reconstructed. This leads to extremely strong bones that are very dense and heavier. It's not limited to weight lifting it's just, putting stress on the bones (good stress like walking/running or calisthenics and weight lifting, not stress to induce fractures or anything like that) makes them stronger. This is very important as a lot of times muscle strength exceeds bone strength and you end up with all kinds of problems...especially joints, I can't recall the name of the vitamin or supplement, but you can find it any(should) drug store, it's...just something that helps keep the cartilidge and joints strong and well lubicrated.
As for calories and what not, 3,500 calories = 1 lb of fat. I believe 1 carb = 7 calories, and 1 protein = 4 calories. Fat in the body is 9 calories I think. I could be wrong but basically, if your body doesn't have enough protein your muscles will break down and you will be losing strength and muscle rather then building and gaining. If you have excess protein it will just be turned into a calorie. Though it is easier for the body to digest and process carbohydrates for energy, so it is suggested that you figure out your daily needs.
To estimate your daily calorie needs the equation is:
Height: __ Ft. __ In.
Women: 100 Body Mass Units for first 5 feet of height and +5 BMU for each additional inch.
Men: 106 BMU for first 5 feet of height and +6 BMU for each additional inch.
Total BMU is? _____
Select Activity factor: Sedentary=13; Active=15;Very Active=17
Multiply BMU total by Activity factor, total is your daily Nutritional Caloric intake value.
This is an estimate and not an exact figure.
Mine would be, 5'10", 166 BMUx15 AF, 2490 Calories a day to maintain body weight.
I had a much better way to estimate the calories, based on height, weight, and activity but I no longer have that sheet, the number is roughly the same though for me. Granted I am of average weight now at 160-165. If you are heavier and work more, you would obviously need more calories.
For the vitamins, you have to be careful that you don't take an excess amount, I can't recall but there are a few vitamins that if you take too much it can cause A LOT of problems, so it is suggested and recommended that you only take as much as you require. This might take a little effort on your part to look at what your eating and what you are taking in the way of supplements, as some vitamins add like say, zinc+calcium, and then you take a multivitamin that has 100% zinc for a 2,200 calorie diet, and another vitamin that has zinc+?. I think the excess zinc just makes your poo a little harder lol...

But there are some (most I believe) that have no problem and will just get pee'd out (dark yellow urine).
I believe B6 and B12 are the big ones for muscle gain. I think it was aristotle who said, it's the mean between two vices? You want an adequate amount, not an excess and not a deficiency in anything.
I hope this helped a little. I realize it's long but this subject is huge so...tried to get the basics in.