OK, being an empiricist and a moron, I propped my bike up, sat sidesaddle on the rack as pictured, and bounced my ass around to test it. Thing felt solid, and I weigh about 160. I'd say that's a pretty decent limit at which either A) the rack will suffice to carry your ladyfriend, or B) you need a lighter ladyfriend.

I most certainly wouldn't go fast if I did decide to try it, and I'd prefer not to ride in traffic. But if you had to do it, it might work. The pics from the Netherlands suggest that it's not entirely fatal.

Lest I open myself up to legal liability, I cannot condone such stupidity, though I do enjoy engaging in it myself.