What's your height, current weight, and general bodyfat (not a percentage, just a description - skinny, lean, a little muscular, sorta fat, etc.)?
What, exactly, did you eat yesterday?
Mass gainers are just liquid calories, the same as eating food. If your friend took a shake and gained fat (depending on how much fat it really was, some fat is certainly expected when gaining size), it's no different than if he was just eating too much steak and potatoes.
A good training program and smart nutrition.
If you're inconsistent in your nutrition, you'll get inconsistent results. Period. If shakes (like a weight gainer) help you with consistency, then they'll be useful, but really, lots of people are busy and still manage to eat well. A bit of planning will do wonders.
That's your choice, but Plazma is pretty much the most effective workout shake around. A bunch of people have been using it and writing about their results.