K.M.
I found that the best gifts are the simple things. Like a set of pens or colorful pencils. She shouldn't give candy since it will lead to bad habits and tooth decay. It shouldn't be a surprise if her hosts decline the gift, but she should attempt to give it to them a few times. She should give the gifts in private and not in a big group where others (who won't get a gift) are around. Also, she should consider wrapping the gift if possible if she is buying the items in the States before departing.
Sometimes the best gifts are those that are chosen after she finds out what they might need. If she spends enough time with them, she could figure out what might be helpful - maybe a container of flour or sugar. Some new kitchen supplies. A hair barrette or clip for their daughter?
The best gift I gave was a small container of Tiger Balm to a porter on the Inca Trail. I walked out of my tent and found the 20 year old grasping his battered feet in pain. The Peruvian porters carry 40 pounds of gear the entire 30 miles of the Inca Trail... at least five times a month! Most of them make more than Peruvian teachers or doctors (but that isn't saying that they make a lot in global terms) and they aren't treated well by their bosses. Worst of all, they wear sandals made of old bicycle tires! The smile on his face when he first experienced the deep heat soothing of the balm will stay with me forever. He soon shared it with the other porters and the container was empty in just a day!
I hope this helps.