My friend smokes weed to help herself concentrate on homework. she said there's a lot of advantages. is that true? how can i convince her to stop?

Maybe anxiety issue: Maybe the weed calms her down so she can study. Weed clearly decreases cognition, so maybe she thinks too much and weed helps her not think about things. Generally weed causes decreased judgment, analysis and decision making.

Not likely: Marijuana has lots of effects - euphoria, increased appetite, etc., but increasing concentration is not one of them. Your friend is fooling herself and using this as an excuse to smoke pot.