First make sure she checks with her OB/Gyn.
Second, with her docs approval, make sure she gets "graduated compression" socks with the least ammount of compression. It should be around 10-15 mm Hg of compression. There are maternity specific stockings pantyhose as well. DO NOT GET THE ANTI-EMBOLISM SOCKS!
Some of the more common brand names are Jobst, Activa, TEDs, and Futuro.
I recommend going to a home medical store that has a fitter that specializes in these types of socks.