I started taking creatine about 3 months ago - 2 months on and one month off, and I have now started taking it again. Creatine most definitely will not cause you to gain weight, in fact I believe it has no direct effect on your weight at all.
I'm by no means an expert on the supplement, but the main effects of Creatine are that it will give you an increased recovery time after workouts, allow you to do more/go for longer in a workout session, and allow more water to enter your muscle tissue (you should increase your water intake).
Your friend says he has become alot stronger since taking creatine for three weeks thats pretty good going, but I doubt it is a direct effect of the creatine. I suspect instead it is due to more frequent, higher intensity workouts enabled by the creatine - which in turn could account for the weight loss.
I don't know what your current workout routine is so can't comment on if you should or should take this supplement. From my point of view I previously was gyming twice a week for weights, Monday and Thursday - this is the length of time I felt I needed to recover before another session. On Creatine I am finding it easier to go three times a week Mon, Wed, Fri and recover nicely in between.