In answer to question #1 ... no. Cycling builds muscle, muscle gets bulky.
In fact, nothing she does in terms of exercise will cause her legs to slim down ... everything she does will build muscle to some degree or another. There is a very common misconception among women that the more leg exercises they do, the slimmer their legs will be. Quite the opposite in fact!
I just did a quick calculation. If she is 6'1" and 155 lbs, her BMI is quite low. Therefore she can't have thighs and butt that are all that heavy ... she hasn't got the weight for that! What cycling can do for her is to help develop muscle definition ... and then it is up to you to tell her she has "hot legs".