It means that she is going to eat this food as a favor to him or because he asked her to, but it's going to make her throw up. In this way, she's going to throw up for him (meaning she's going to eat something gross, because he asked her to, and then she's probably going to throw up). It implies that she doesn't want to eat it because it's disgusting, but she'll do it for him because she likes him/he's a friend.