Yeah i had one (actually had many) like that..
He was my "friend" as long as i treated him to food from the canteen and helped him in his studies..
And when i stopped he started insulting me, calling me names and revealed all the secrets i had told him..
Before I started going to a private school almost everyone I ever worked with used me to do their work for them. Apparently they thought I was a 'nerd' because I was smarter than them and afraid of failing, so I must enjoy stressing over schoolwork. I often ended up doing the entire project for fear of getting a bad grade. One of the girls I went to elementary school with, who always acted too bubbly and fake around me, worked on a huge project with me and some other kids where we had to present in front of the class and some members of the government. She not only made me do all the work, but she stole my notes from my desk the Friday before. I was frantic all weekend and my mom was really frustrated with me because she thought I forgot them. Other people have pretended to be friends with me even though they were brats or morons, yet they feel the need to be creepy and over-friendly with me. I still hold grudges on those people. They suck.