For some people this can be a daunting situation, and many are not sure of what they can or should do. Make assumptions about the student. Often people exhibit behaviour or actions over which they have no control. This is the illness, not them. Presume to have all the answers.
Many illnesses are complex and require professional help.
Often it is a great support just to have someone there. Ask what you can do. Help students to make full use of university services that are there to assist them. The Disability Officer and Counselling Services offer a wide range of services. Offer practical assistance.