Please pass along to your girlfriend...
I was a summer associate 3 times (2x with same firm, 1x with different firm) and I've worked at a big firm in the US and a different big firm in Europe (also spent time at a stellar firm in Africa...great experience). I guess I've been around that block a time or two! (I've since quit - really hated THAT rat race, but I digress - the point of another chain).
What worked for me, as a summer and as an associate and beyond (you do plan to continue in the profession, right??), is to think maximum use, low (not necessarily lowest) cost. Consider what dress shoes (recommend no peep toes, what plain heels do you have?) you already have and go from there. Do these go best with a grey suit, navy suit, black suit? I always recommend two suit sets (jacket with pants and skirt) in two basic colors - no crazy prints/patterns, as you also want to wear them for 10 years out. DO NOT buy suits where the style of the jacket must stay on - those are church clothes and not office clothes. You want to be able to wear different shirts underneath to dress the combo up or down - you probably have enough interesting shirts already to get by here. With lawyer desk jobs, women - like men - take the jackets off and throw them on for lunch or when meeting clients or partners. With that said, I got by A LOT using my skirts and pants and pairing with sweater sets or dress shirt and vest or sweater and scarf (brought jacket in bag, in case needed). So the two suits, with shirts and shoes already owned, will do it -- for two summers AND for start of first job. Besides, you must have a few nice skirts and pants already that can fill in between while the suits are getting dry cleaned. AND if you pick the right suits with totally versatile jackets, you can wear the jackets with your already owned clothes, too (black jacket with grey pants and vice versa is also a classic combo, although not the dressiest). And why not a jacket with jeans after the summer is over for a special date night??
I never paid high price for my suits, but was always scoping for finds. I looked at random places and always found something. One suit I found for super cheap at an outlet mall and the inside lining is now in tatters, but it sooooooo classic and looks so good that I will never get rid of it! Anyways, the tips on consignment + tailoring are pretty good...but it might be difficult to find the super classic suit combos there - who would ever buy those and get rid of those? Not ladies who lunch. Try an outlet mall or even suck it up and go to Macys and shell out. Once you have the basics purchased, THEN you can go scavaging for add-ons.
Never go too cheap in an upscale law firm environment - partners, managing associates and clients can smell it a mile away. On the other hand, I never wore anything very expensive and also didn't suffer. (Never paid more than $100/150 for any suit combo...and no accessory more than $80, and that includes quality, attractive shoes.) Yet I did see a few snafus over the years...like skirt too short, v-neck too deep and so on. Law firmers love gossip - don't be the source unless it's about a promotion or bonus or new client!
If you go for a classic, clean look, you'll never go wrong...and ultimately save money and time in the long run! Good luck!