You don't say how far along she is (that definitely changes things)...she should be calling her doctor to let them know, but some women really swell up with summer babies. I am due with number 2 in less than 6 weeks and have had zero swelling, yay!, but with my first, who was also an August baby, I looked like my face and feet were going to pop by the end of June! It could be gestational diabetes or pre-eclampsia, but she would have to talk to the doctor and be seen to be sure.
EDIT: At this point in time, if she didn't have gestational diabetes when they tested her, she shouldn't have it now. Pre-eclampsia is definitely something to be concerned with. Have her call her doctor...she could definitely be retaining too much water!