Hi and welcome to the forum! I am glad that I can help you.
You want to know the cause of the symptoms that your partner experiences.
Nausea may appear in the following sutuations: hunger, gastritis, intestinal infections, anorexia, and pregnancy in women.
Given the data that you described, your partner may experience nausea due to any of the reasons listed above. He may consider having breakfast after waking up, to avoid hunger and irritation of the gastric lining due to the effect of hydrochloric acid. He may also consider avoiding stress because it may increase the secretion of hydrochloric acid and may increase the irritation on the gastric lining. If the nausea is accompanied with vomiting or diarrhea, he should have a bacterial culture from a stool sample taken to rule out possible intestinal infection.
Please keep in mind that I provide medical information only. I am not able to diagnose medical conditions online. Please contact your doctor for further advice and information about diagnosis and treatment options.