If you're too nervous to cut 6", just cut 2". Then another 2", and so on. If you send her to a salon, she'll come back waist length or mid-back... or worse.

I would recommend cutting to bottom of shorts. Yes, you lose the 2 years of growth, but then you can review how she cares for her hair, what might be causing the thinning: lack of nutrition, physical damage to the hair from brushing, etc.