I can see the appeal of the OP’s approach. He can take maybe 3 or 4000 grafts via FUE then he may think, “Hell, I have maybe 4000 grafts left but it looks like I need a helluva lot more”, in which case he can abort the whole process and not be left with a smiley. Plus, any decent FUE surgeon knows enough about strip to know where the strip would come from so they could leave it unharvested.
That said, if he’s determined to do both and he’s committed, it would make sense to get stripped out first. That way, the final appearance of the scar is known (so the FUE surgeon can not harvest around it so it’s easier to hide, if necessary) and any areas that have thinned because of the scalp being stretched can also be ignored, so FUE grafts are only taken from where they can really be spared. Doing FUE first does run the risk of thinning an area to the point where it’s just about okay, but then stripping and stretching the donor could push it over the edge to sparseness, by which time it’s too late to do anything AND you have a strip scar to think about.
So there’s advantages and risks to both approaches. Depends on his NW level. At the end of the day, to be safe, I’d go with, “Is it possible for strip to give me the result I want?” If yes, then go for it. Those extra FUE graft will be a nice bonus on top of that. But if he’s just trying to max out his graft numbers because he’s a high NW, he could be taking quite a large gamble by getting a scar AND thinning his donor which would be a real lose-lose situation.