to the first question... dried shrooms can keep for a long time if they are stored right. they can definatly last 3 weeks if she kept em dry and in a dark place.

to the second question... you should always eat shrooms on an empty stomach, and her weight might cause her to trip harder but it is not always true that smaller people trip harder, it depends on the type of mushrooms, how much you eat, etc, etc.

and an 8th isnt that much really in my opinion she should eat the whole 8th but thats just me.

and make sure she smokes some weed after she eats em but before she starts trippin... itll chill her out before the trip