I have a lady friend who's interested in getting into road cycling. The smallest frame sizes I've seen so far in the market are 48 and, in one instance, a 47. Is this about the most common and/or smallest starting point for frames for these riders? She's average weight (around 100lbs). Any recommendations on frame size would be appreciated.
My wife is 5' 2" and raced elite road for many years. For her proportions 48-49 cm frames seemed to work well. Sometime she had mentioned interest in a smaller frame, but when I looked at how she fit on the bike I think it was due to flexibility issues (at that time) rather than frame sizing issues.
Of course this is one person's experience and therefore not a canonical answer. There are a lot of factors, other than frame size, that govern fit, but 48/49cm is likely a good starting point for someone 5'2".
NOTE: different brands will have slightly different fit dimensions (stack and reach) for the same quoted frame size. Frame sizing is not standardized.
You have to try the different frame sizes. I'd suggest looking at 650b or 26" wheel sizes if possible though, or possibly a kids frame -- I would not recommend using 700c wheels, as is standard on road bikes, for someone under maybe 5'5". Some of the women's specific marked bikes go a bit lower in size than the mens bikes and most major manufacturers (certainly Trek/Specialized/Giant) produce them.
Different frame sizes work for different people of the same height (depends on leg length, torso, arm length, etc.) and frame sizes aren't consistent between models of bike. So she has to try a bunch of bikes.
Pay attention to the effective tube length (ETT, VTT and whatever it might be called) which is the horizontal length from the handlebar to the saddle tube. While saddle height ('conventional' size) can be changed easily, ETT is hard to adjust.
The easiest way to determine the right ETT is a calculator.