Thailand has a number of dishes that would suit your father-in-law.
There are a number of simple grilled pork dishes such as khor mu yangkhor mu yang (grilled pork neck, thoughtthough you might leave the translation as grilled pork), mu ping mu ping (pork on a skewer), satay mu satay mu (pork satay). There are stir fried dishes such as pork with garlic and/or black pepper. Roasted and roasted pork dishes such as mu kopmu kop with a crispy salted outside and moist meat inside.
Thais love chicken, so fried, grilled and rotisserie chicken are available country wide.
And there are plenty of non-spicy dishes available, such as noodle soups, fried noodles (like pad thaipad thai, pad seyupad seyu), fried rice dishes, stir fried vegies like asparagus and shrimp, mixed vegies, mild soups like Tom Kha Gaitom kha gai (chicken soup), etc.
They will find similar dishes in Cambodia and Vietnam as well, though with different names. As they are on a guided tour, their guide should be able to steer them towards specific foods that are not spicy in Cambodia ad Vietnam. They could also get their guide to write a note "I can not eat spicy foods, please do not use any peppers" in the appropriate language for each country.