Another NO vote on the Sportster.
My recommendation for a young novice rider would be a used Yamaha 250 Virago or a V-Star 250 (same thing but newer version). The seat height is listed as 27 inches and the dry weight as 302 lbs. The engine is a V-twin and it sounds like a small Harley. Check out this review, then go to youtube and you can hear what they sound like.
2008 Yamaha V Star 250 | motorcycle review @ Top Speed
I don't think that the Sporster is necessarily top heavy, but it is just plain heavy. My '08 Sporster 883 C (that I have for sale) weighs about 600 lbs, heck my '06 Mastiff only weighs 800.
Stephanie was real nervous on the Sporster, and she's 5'6 and 150 lbs. She dropped it once trying to back up.
It wasn't really what I was looking for when she decided she wanted to start back riding. I was looking for a Honda Rebel or something like a Johnny Pag Chopper (which is like a Chinese copy of a Honda Rebel). But I found a good deal on the Sporster and bought it.
Then back in Early December I found and bought an '08 Rhino Hunter S. It's like a Chinese copy of a Big Dog Mastiff frame with a Chinese copy of a Yamaha 250 V-twin motor in it. It only weighs only 420 lbs. She likes it so much better it isn't funny. I don't have any pics with my Mastiff yet but here it is beside the Sporster.
The seat height of the Sporster is listed as 27 inches, the Rhino is listed as 26.5 inches, but the big difference is in how wide the bikes are, the Sporster is so much wider you need longer legs to reach the ground.