I make machine sewn kilts very similar in construction to the one you describe so perhaps if I describe what I do to re-size one of my own kilts when a customer loses a lot of weight it will help you to understand the process.

To insure that the aprons continue to look and hang correctly measure the apron and re-mark as necessary using the following criteria.

The apron at the waist should be slightly larger than half the waist measurement.
The apron at the hip should be slightly smaller than half the waist measurement.
By slightly I mean approx the width of one pleat.
The edges of the apron should form a slight "S" curve. Straight from the waistband to the top strap - then flaring out to the hip - and continuing down to the hem. This curve is to allow the apron to lay flat when standing and walking and not curl outwards.

Now to the size adjustment.

The way I resize a Kilt is similar to the way the kilt was originally made. Imagine that you have the kilt laying out without the pleats pressed in or the aprons folded. You will have a long piece of fabric with a join in the middle. If you wanted to make your piece of fabric shorter you could un-stitch the join and move one piece of fabric over and re-join instead of cutting somewhere else and creating two joins.

1. Remove the three straps and their buckles and put them aside.

2. Remove the waistband by un-stitching it. Set the waistband aside.

3. Measure the aprons and adjust their size if necessary. The adjustment is done by finding the center of the current apron and measuring from the center outwards to each side.
If you need to adjust the size of the aprons a significant amount you may also need to remove the fringe on the right side of the outer apron.

4. Using your customers waist measurement you need to calculate how much to remove from the kilt.
Take the customers waist measurement and subtract the width of the apron. Then measure that amount along the pleated part of the kilt. This will tell you how many pleats you are going to remove.

5. When you are ready you need to un-stitch the pleats of the kilt near the current join in the fabric. You may need to un-stitch the pleats for quite away due to the overlap in the tapered area. Once you are able to lay the fabric out and look at the current join you will understand how the join is done you are ready to make the size adjustment.
Take caution here as you want to sew the kilt back together so that the join of your pieces is not noticeable. You can look at the existing join to see how this is done. Then un-stitch the existing join.
You will now have two pieces of kilt. One with the under apron and some pleats and one with the outer apron and some pleats.

6. I find it helpful to un-stitch and iron out the creases of the pleats I am removing so that I can align the fabric where I will join the kilt back together.

7. I also find it helpful, to avoid a mistake, to pin the kilt together at the join and temporarily fold the pleats back into place. This way I can measure the kilt to insure it is the correct size and to insure that my join is in the correct place.

8. Once you confirm the kilt is the correct size you can sew a new join into the fabric. This will result in a section of fabric that is excess at the join. This excess can be cut away and saved for your customer to use if the kilt ever needs to be let out again or if they wish to make flashes.

(A hint on measuring the kilt to confirm the correct size. --- Measure the Kilt along the line where the straps and buckles are located. Measure from the right edge of the outer apron (where the fringe is) across the back of the kilt to the right edge of the under apron. Because the outer apron overlaps the under apron you do not measure both aprons just the width of outer one and the width of the pleats.)

9. You can now put the kilt back together. Simply sew the pleats back down in the same location that they were originally.

10. Re-install the straps and buckles.

11. Re-install the waistband.

You stated that you have the skills to handle this job so I am assuming that if the general process is described and you have a chance to look at how the kilt was originally made you will be able to figure out how to re-size this one.
If you have any more questions please feel free to send me an e-mail to
steve@freedomkilts.com
or you can send me a Private Message via this forum.

Good luck.