650b for small rider? I'm considering getting a bike for my friend; she is interested in riding road/gravel with me, doesn't own a bike of her own, and doesn't know too much about what's available.  

She is about 5 feet tall (1.5 meters) and 100 pounds (45 kilos).  I believe a 47-49cm frame would be a good fit for her.  Considering her small stature and the small frame size I was thinking about a set of 650b wheels.

It seems there has been a trend of 650b fitted bikes recently and that the wheel size is slowly creeping back into the market.  One of the advantages is that one can fit a wider, taller tire without worrying about clearance or modifying frame geometry. I got to thinking that outfitting her with a 650b wheel set might be a viable option with some advantages over a 700c wheel set.

My question is what are the advantages and disadvantages of fitting a petite rider with a 650b wheel set, in comparison to a 700c set.  I'm especially interested in bike handling, tire fitting, and frame geometry implications and considerations.
I personally am a fan of the 650b movement. With a 650x47b tire, you get approximately the same rollout diameter that you get with a 700x28c. So as far as the distance per revolution, you are about the same with a 650bx47 as you would be on most road bikes. The benefits start with increased air volume. This leads to a much softer ride.

If you are talking about a conversion from a 700c to a 650b on a bike, there are very interesting things that happen to the geometry. Your bb drop will be greater, so clearance could become an issue. with a lower bb, however, comes a lower center of gravity. This could increase stability through corners. Another consideration is that most bikes designed for a certain sized tire. By changing that, you could affect things such as trail. A decrease in trail would help in low speed handling, but could greatly hinder higher speed stability. 

One last consideration for converting a 700c to 650b is braking. If the bike has discs, then no worries. If they are v or canti brakes, then the reach may not be great enough to have the pads make contact with the braking surface on the wheel.

http://ridinggravel.com/components/wheel-comparison-650b-vs-700c/

There are some bikes road type bikes that are designed with the 650b in mind. Some are children's bikes, but if the rider is small, a children's bike could be easier to control.