My frienc keep throwing up while cooking in a kitchen. Why?

A friend of mine went to school to be a chef and when he finally got a job cooking, he kept getting sick whenever he was in the kitchen. First, he worked at Applebees, and told me that he would often feel nauseous when he was cooking food, and they had to let him go because he kept throwing up and having to be sent home. Recently, he started working at burger king to get some money, but even there he keeps getting sick. While in school, he told me that he would sometimes get nauseous when he would eat some of the food they cooked, but I dont think he got sick simply by being near the food like now. Can anyone please help? My suspicion is that he may be allergic to something in the kitchen or he gets sick from the smell of grease. Any help would be appreciated