I would wager your girlfriend is plenty strong enough to handle a 6lb rifle, she just hasn't mastered said "handling" skills. Practice will solve her problem if she's serious about learning to shoot well.
+1 for the Mini 14, my tactical (non folding stock model) weighs in at 6.5 pounds. This is the handiest true rifle carbine I've ever handled, and yes, I did at one time own an AR (DPMS AP4). Reliable, durable (no alluminum), AND ACCURATE.