Personally, I'd call ScubaToys and tell them what you're wanting to do. Order the medium (like gibby suggests) making sure they understand you may need to swap something out...the shoulders being a very common piece that needs changing.
When your girlfriend opens and tries on her new BC, make sure she knows how to adjust and wear it properly. The middle straps should sit at her natural waist (below the ribs), and the BC should be tightened from the bottom up. Once all straps are loose and the corset is zipped up, pull the BC down until the middle strap is at waist level. Tighten the lower straps first, then middle, then snug the shoulders down (not tight!).
The straps should have 40-50% left for thicker/thinner suits. If any of them are maxed out either in or out, you'll need to swap out parts. If they are ALL pulled in completely, she'll need a smaller size.
This process should take no more than 10-15 seconds, and I've found being horizontal is the best position to get the perfect fit. Once adjusted properly, I never have to touch my BC during a dive and can literally be in any position without worrying about tank roll or ride-up.