At 5 foot 9 she can likely shoulder most guns out there as long as they're adjustable. I've noticed that girls interested in airsoft sometimes*misjudge what their restrictions will be in terms of gun length and weight and, I quote, "want something small". My wife, who weighs in 90lbs and 5 feet tall made the same decision -- MP5 -- but in the end she found she could properly hold my M4s with the stocks properly adjusted.
In this regard, a shorter female will do well with an M4 that has a stock that can be adjusted, unlike with the MP5. Note that many of the MP5s on the market, with the exception of the plastic Classic Army MP5K, are actually really heavy compared to many M4s.
My suggestion is a stubby M4 or perhaps the KAC PDW, which is very light and has a really comfortable center of gravity for ladies.
Also, I was able to find a great Condor rig for my wife that fit her just fine, PM me if you're interested in which one because I'll have to dig up the product name.
Now you know