Northern Thai restaurant in BKK?
was there some specific dish your husband was looking for? if you are just looking for basic khao soy and chiang mai sausage, there are a handful of places in the city that serve them. for khao soy look in food courts (the less touristy the better tho the pantip plaza food court has it). there's also a restaurant on soi 20 that serves thai food from all regions and has some northern thai dishes, including khao soy. sorry, i've totally blanked on the name of it. not spectacular but pretty good, and the only place i can think of that's open for dinner and is a real sit-down restaurant.
probably the best khao soy is at a the lunch market on lang suan soi 6. it's only open weekdays for lunch tho.