I gave this a lot of thought, especially since this is SUCH a common situation. While she won't be able to avoid shopping entirely, there are lots of pieces that will transition nicely as she shrinks since they're not intended to be form fitting. Here are some options to consider:
What You'll Need:
A great-fitting pair of straight-leg jeans and/or black pants: If your early maternity jeans still fit and aren't saggy, then wear them, but otherwise you need to try on jeans until you find a dark wash pair with a straight cut that you can dress up or down. This cut is going to look better than a flare, although you can also do a bootcut if it makes you more comfortable. If your office isn't totally casual like where Liz and I work, you'll need some great black pants as well. I recommend trying on different styles at TJ Maxx or Nordstrom Rack, or checking eBay if you don't want to pay full price. And when you get home, cut out the label if it makes you too upset. Nothing makes people look larger than ill-fitting pants.
A black maxi dress: You can layer this with cardigans and a denim jacket that you already own for work, and wear alone as well. A crossover front will work well for pumping, and you can keep wearing it as you get smaller since the cut is so forgiving.
Completer pieces: I'm talking about cardigans, blazers, and denim jackets. You probably have these already, and they'll help you get more mileage out of basic black and white tanks and tees.
Nursing-friendly tees and tanks: Although they're not cheap, go ahead and invest in a black and white nursing tank/tee, but also look for non-maternity styles that are intended to be flowy like the option from LOFT above.
A fun printed skirt or dress: To mix things up a bit, look for an inexpensive printed skirt or dress that fits you as you are now. It'll make you feel girly and pretty, and you can sell it on eBay when it gets too big.
Colorful Accessories: Since you'll be repeating outfits more than you normally would during this time, look for some inexpensive statement necklaces that will add color to your outfits as well as some fun but comfortable shoes that will make you feel good. And, if you don't already have one, get a great diaper/pumping bag that looks good when a grown woman is carrying it around. You'll get as much wear out of it as you will a new purse (maybe more!) and you should enjoy carrying it.
Okay mamas, I know you ladies probably have some great suggestions for Liz. Let us know what pieces you loved in the first few months after delivery!