I shoot two 24" barrel AR rifles.
The rifle would be no more unbalanced with a collapsable stock than they are with a standard stock and some of the newer design collapsing stocks are actually heavier than a standard fixed stock set up.
If you use a bi-pod the rifle would only be more comfortable for your wife to shoot with a collapsing stock in place though I admit the set-up may look a bit goofy.
One of my rifles uses a Bushmaster 1-9 twist, 24" stainless Varminteer barrel, an Accuracy Speaks free float tube and a 6X42 fixed power scope mounted on a flat top receiver.
The other rifle uses a 1-10 twist Olympic 24" barrel with a Pachmayer float tube and a 6X24X50 variable scope on a flat top.
I sometimes use a Harris Bi-pod on both rifles and both have been shot in competition, (300 meter free rifle), and both are used for hunting.
Both rifles will shoot under MOA to 300 meters, neither requires a flash hider or muzzle brake and I recommend a 24" barrel for general shooting and hunting.
If you want a heavy,stable rifle you could use a shorter M16A1 fixed stock which should be short enough to comfortably accomodate your wifes smallertature and place a lead weight in the storage area of the buttstock.
This will bring the rifle up to 11 pounds or more depending on the weight of the scope, mount,s and bi-pod and make for a very stable bench rested or prone position shooting rifle. HTH