DH grew up eating plenty of peanut butter and never noticed any problems from it. He continues to eat it without any problems. Is there any reason to think he could suddenly have a severe reaction like the allergist claimed?
Maybe.
Just some general information, not intended as advice (see my signature line):
The type of reaction in question (the very serious kind) is called anaphylaxis. It's not common but not so rare as to be negligible. Fatal reactions are most common in people in their 20s.
The pin-prick test will "pick up" about 90% true peanut allergies, so a negative result is reassuring, if not a total guarantee. A positive result leaves you with a greater chance of a future peanut reaction (about 1.8 times greater than the general population). If the reaction to the skin test (a "wheal" like a hive) is strong, the odds go up. Prior exposure in the absence of symptoms does not eliminate the risk of a future reaction.
The deal with nighttime versus day is simply that if it happens while awake, there is a greater chance to reverse the reaction with epinephrine injections. The allergest making the recommendation likely also advised that he carry an epi-pen for that purpose.
If I had a large wheal reaction to peanuts -- even in the absence of problems in the past -- I would avoid peanuts (or at least only eat them if my trusty epinephrine was at hand) and carry an epi-pen in any situation where I might inadvertently be exposed to peanuts in food or being cooked (vapors). Chances are high that it will never get used, but if you need it and don't have it, the outcome could be serious or fatal.
Fortunately, such reactions are not common in my practice, though I have seen a couple. One young guy had a reaction while in the act of eating the peanuts and in the ensuing commotion he aspirated the peanut and had to have it bronchoscopically removed right after we stabilized him.
Sounds like you might benefit from discussing this with your DH's doctors since every case is different, and generalities may not apply to him.
Hope that's helpful.