telling a friend no more...

I was asked by a friend to watch her baby who is 3 months old. Mine is 11 months. This baby is SO colicky and gassy and fussy. It's to the point now where I just don't think I can handle it anymore. What should I do? She pays me $200 a week and we need the money, but I'm more concerned about her baby. I think he misses his mommy and I just don't think he likes me. She won't give him medicine and he is very gassy. We changed his bottle and I even started coming to their house with my baby to see if it would help him. I don't want it to affect our friendship, I just do not know what to do. Should I stick it out? How could I make this easier for the colicky baby? Is there a way I can tell her that I can't watch him anymore without hurting anyone's feelings?