everyone else will tell you to push through. but being severely sick means that you are having a severe detox reaction. so that also means you are putting a severe strain on your filtering organs. or that your girlfriend can not process the chemicals that occur in the body when fasting.
do a precleanse diet for a month (totally vegan and no processed stuff)
most fasting guides say a week will do...
but in my experience a month of a precleanse diet makes The Master Cleanse actually feel good! i had alot of sniffles and nose blowing..sore throat from post nasal drip. but i was not sick at all.and i didnt even do the SWF. only one tea every three days.
even if you only go vegan for a week and come back to the cleanse it will make a difference. someone that is really toxic should not under go a long fast. instead they should eat cleaning and do a series of shorter fasts until the body is built up enough and detoxed some so it can better handle the long term fasting.