I use a few yards of material and have them (and their partner) tie it in the back. I also give them little white squeeze clips that I bought at a grocery store (almost like a chip clip but smaller). They hold pretty well and super easy to clone out if one sneaks into a shot. I also have them use them on the back of the skirt.
Tell them to make sure they feel too tight. The material I use slips a bit...the curtain material might be fine though.
Also, I almost always see a bit that should be covered up...most preg. mom's have gotten over a bit of their modesty after the 20th dr's appt.

Don't be nervous and embarrased, just help her fix things. I've had mom's be a bit uncomfortable but as soon as I quickly helped them get it fixed, it was really no big deal.