Ken,
As I'm currently hip-deep in US Navy and Marine aviation in WWI, I can give you my best guess:
If he was over there in 1917, and in the Dunkirk area, then he was probably assigned to NAS Dunkirk, one of the first stations to be taken over by the Navy. Off the top of my head, I don't think it was commissioned as a US station until the beginning of 1918, but US personnel were there before then. It was a seaplane station for attacking U-boats going in and out of Belgian ports. They flew Donnet-Denhaut flying boats and Hanriot floatplanes to defend the DD's against German aircraft in the area. NAS Dunkirk never upgraded to Curtiss HS-1's or -2's when they became available as there was not enough space in the harbor to fly them out.
There was an RNAS air group in the area. After amalgamation, it was designated RAF No. 5 Group, and included squadrons 211 (DH-4's and later -9's), 213 (Camels), 214 and 215 (Handley-Page O/100's and eventually O/400's), 217 (DH-4's), and 218 (DH-9's). If I remember correctly, it was No. 211 that flew seaplanes out of Dunkirk before transitioning to landplanes.
NAS Dunkirk apparently provided a lot of assistance to the RNAS/RAF Group there.
After the US Navy authorized the Northern Bombing Group on April 30, 1918, Navy personnel began arriving in the Dunkirk area. It's possible your wife's grandfather was transferred to the NBG. Pilots didn't start showing up on the NBG's fields until mid-June, and it was August before the first planes arrived. (NBG's problems with planes is a story unto itself!) In the meantime, many US Navy personnel were formally attached to the RNAS for training and to keep them busy while waiting to begin independent operations. The NBG was supposed to bomb the U-boat pens at Ostend, Zebrugge, and Bruges around the clock, but on November 1 (I think), the Germans abandoned those bases, and the NBG began assisting No. 5 Group in supporting the armies at the northern end of the lines. At the time of the Armistice, the Germans had retreated far enough that the NBG was moving forward to be closer to their new targets.
By that time, if I understand correctly, all the RAF units in No. 5 Group had been transferred to the "Independent Force," No. 81 Wing. I'm not sure what happened to them after that, but you can look them up on this RAF site:
RAF - Historic Squadrons
Now I have a question for you: do you know the names of any of the airfields that they were moving hangars to? I know the Dunkirk seaplanes (by this time attached to the NBG command) were moving to Zeebrugge, and the Day Bomber Wings of the NBG (almost all Marines) were moving to Knesselare, but the Night Bombing Wings (all Navy) were going to someplace called "Marin Alta," and I can't find anything more about it. I have found a "Maria Aalter" near modern Knesselare, but I'm not sure if that's where they were going or if it refers to someplace else.
-Larry Burke