Husband with psoriasis, what are common triggers?

My husband, 47, has psoriasis in his scalp, ears and face. It started 4 years ago, when we had our second child and he was building his own business. It is a stressful life; 2 little kids, we both have demanding jobs and we live in an expensive city. I see that it flares up when he is stressed (say we had a fight, he had an intense work day, the kids are difficult, we talk about our finances, etc.).

Question is: What are common triggers? Is it stress, diet, both, something else? He's not eating spicy food and is only on one coffee per day. He barely drinks and doesn't smoke.

How can I help him spot his triggers? Please help!