The Jewish dietary laws known as Kashrut are extremely complex. If you are caring for a person who follows these restrictions, there is no substitute for a detailed talk with them about what they do and do not eat. Some religious Jews follow these regulations strictly, some partially, while many do not observe them at all or observe them only on special religious holy days.
To “keep kosher:”
For most other foods, various kosher handling rules apply. When shopping, look for the label “kosher” or the letters “K” or “U” within a circle.