Yes! Opinions will vary about whether or not it's "fly fishing" or not. If the guy is using any tackle other than a fly rod, then IMO he's simply fishing with a fly, not fly fishing. It's not an uncommon practice. The power bait thing is not fly fishing, it's lure or bait fishing & there again opinions vary on it.
When I was tying flies commercially, there's a shad fly I tie that a local shop was selling the heck out of them. The owner told me that the majority were being sold to guys who used them with spinning tackle for Perch, not fly tackle. Not "fly fishing", but again fishing with a "fly". There's a difference.
Whatever works, as long as it's legal. Here in MD, the only concern about "fly fishing" is on designated trout streams. The state has a definition of what is "fly fishing" & what is a "fly". Power bait & similar lures are not legal. Even molded plastics are not legal. This is all about what's legal & what's not, & what's enforceable & has nothing to do with opinions.
There are also "artificial only" trout waters & again Power bait is not allowed. MD excludes anything that emits a scent on such trout streams. They define such things as bait. A regular plastic grub is allowed however. On some sections of trout streams where they allow bait, Power bait would be fine.
As far as any other fishing, most anything goes as far as lures & flies. Baits can be tricky, as some things are not legal, (invasive & protected species & such) but generally if it's used commonly as bait, it's OK. MD has a 2 hook per line limit however, it doesn't apply to lures with trebles. Again all in the legal definitions.
IMO, as long as it's legal, if it works, then I see no reason not to try it if you wish. Heck, when I was kid growing up, I tried all kinds of baits with my fly rod, even Power bait. It was almost as much fun catching hoppers, crickets, minnows & crayfish as it was fishing!