I wish I could see what's going on with this a bit more clearly... Can you post a photo of what is around this--is it a waistline?
I've repaired nylon chiffon a couple of times, and it's come out nicely. But, gosh, it depends on if the tear is a "clean" one, how long it is, exactly where it is, and how fine, or not, the nylon chiffon is. Also, just how valuable the item is. Based on what I see in the photos, you might try repairing it--but I can't say for sure if it would be possible unless I physically saw this, or had a clearer photo to look at.
What I've done is find a matching thread and "split" it to make it thinner, as I've never found a thread that is fine gauge enough for nylon (I do this with lace repairs, too, if the lace is very fine). I use the finest hand-sewing needle I have, and, pricking it between the weave of the chiffon, taking tiny stitches, I use an overcast stitch to close the tear. This sounds time consuming, but it's really not--at least not as much as the other solutions available! I would start & end my stitching as near to a rhinestone on either end as possible, or under the edges of the stone if there is some room. That'll "hide" the small knot at the end of the thread. If the "weave" of the chiffon is fairly loose, this works well, as it's a lot like pulling up lace to repair it. And it can be impossible to detect unless one is specifically looking for it. In this case I think the stones will help in camoflaging the repair. If this is in a spot where this area doesn't stick out like a sore thumb, it won't matter if the repair is totally "invisible."
If you've got a steady hand and good touch with a needle & thread, and the other factors are favorable, that is, personally, what I would do. If the tear is more than about 1/2", though, I think I'd just remove the chiffon layer, as Nicole suggests, and leave it that way. Or, if this is a very old, and/or very valuable piece, I'd leave it as is if I were selling it.