I started on a 500 and loved it, but I can tell you, if you haven't read my Kawasaki trip report, I would highly recommend the 250 to any beginner. A 250 is quick enough and nimble enough for ANY beginner rider.
The issue here is that this gentleman is a big guy and wants to ride the bike, as well. The 250 is not going to suit him the same as it is his girlfriend. For me, at 115 lbs, the 250 was quick, easy to ride, light enough to not feel overwhelming...a perfect beginner bike for any female. For a guy who weighs 220 lbs...it might have trouble getting out of its own way.
Since I started on a 500 and loved it, I have no problem recommending a 500 as a first bike for anyone. I think either one is a great choice, but if you are going to ride it as well, you may want to consider the 500 because it'll be easy for her to ride and have enough power for you to enjoy as well.