I think that she would see continued improvement with consistent training in Z2 (~140-150bpm) for many months to come. The cautions against too much Z2 work become more important as speeds increase and the total work / wear & tear / neurological impact increases. Running 20 miles per week is substantially different from running 50 miles per week.
I’d suggest focusing on increasing time and miles below AeT, mobility and flexibility, and strength training (e.g., core workout 2x/week). Taking a conservative approach is excellent. There’s nothing wrong with hiking uphill and running downhill!
One way to gauge progress is to track speed at aerobic threshold. If it’s a 10:45 pace now, there’s room for it to speed up to 10min/mi, 9min/mi, 8min/mi… over the coming months and years.