I'd suggest pinning the pleats at the fell (but not at the waist),
and having her "wear" it held in place at the hips while you
mark or pin the waist. If the fabric is sturdy enough, you
can use masking tape or scotch tape around the waistline
to make quick work of it and pin it without her wearing it.

Once you get it pinned, have her wear it without the
belt around the hips to make sure it is correct, and
adjust if necessary.