@Shugoki and @FeelingHealing are basically spot on with that one. I can’t name any actual weight gain pills, but if they exist, their side effects would probably make them not worth using.
Weight gain shakes and consistent overeating are solid first steps. Drinking calories versus eating them is particularly effective, since you won’t have the same problem of feeling full like you would with food. And by overeating, I mean just pushing a little further, not to the point of being sick. The whole idea being to stretch out a person’s stomach so that, over time, they can eat more and more, upping their daily calorie count.
The time a person eats can also factor in. Eating a big breakfast and a small, early supper is a better way to maintain or lose weight, for example, then having a big supper fairly late at night. The difference is you just took in a large influx of calories just before you slept, so the calories have nothing to do but be converted to fat.
Probably the best option, though, is for the feedee in question to ask another gainer what they did that works. As for the metabolism, I wouldn’t worry too much about that one. It’s very, very rare for a person to have a metabolism that burns so hot that it can deal with all the calories you’re consuming past a certain point. And as someone gets older, everything will start slowing down, making the pounds come on easier.