I think it depends on the different brands of maternity clothes. Most of those clothing sites have sizing charts so she will need to have a tape measure handy so she can decide what size she needs. If the place she is buying it through doesn't have a sizing chart, she could probably look up the individual brand and go to their website to find a sizing chart.

I think that is the hard part about buying maternity clothes on the net. Because they don't have to be as fitted as non-maternity clothes, I find they are bit more forgiving if they are tiny bit too small or a tiny bit too big.