6 inches isn't bod mod at all people. people who exercise and lose 6inches off their body aren't bodmods are they?
when people put on corsets, they don't just tighten it by 6 inches...the body has to adjust, then slowly they decrease the gap. for some it's 20mins, for others it's 2 hours, for others it can take years...again depending on how squishy they are. if she can fully close her 4 22inch corset, then obviously it'll take less time for her to get into her 18 inch one. The thing is, after wearing corset for a long time, it does help reduce the waist, not because of the corset pushing the ribs, but because of the things the person does whilst wearing the corset..so they eat less when wearing it, they stand up striaght increasing good posture, they lose a bit of fat.
if she was wearing it 24/7 that's when you should be worried because that would be in the realms of bod mod. for anyone who wears corsets, they know they can get a bit uncomfortable. i'm sure your gf knows that after every tightening it's going to be a bit uncomfortable because the body has to adjust..it's like wearing shoes a size smaller. she will have no doubt learnt she has to take it easy and won't go as tight next time. i'd also like to point out that although she might have an 18 inch corset,there's every chance that she might not be able to close it fully, just for physical reasons. i would be worried when she begins to 'force' herself to get it fully closed, i.e. when it becomes an issue for her. also finally on the uncomfortable thing, if you see that clearly her tightening the corset is after heR breathing or something, then i suggest you loosen it or make her loosen it. If she's just uncomfortable that it's tight and she can still get on her normal stuff, then she's ok..again it's like wearing something a size smaller, it'll be uncomfortable but it's not deadly.
another thing, just because it wasn't made to her exact measurements, it's still a proper corset, because all the expensive ones use real boning and are slightly 'made to measure' when compared to off the peg corsets.. you may never know though, she might have sent her measurements off to where ever she got it from.