My 16 year old daughter has had shin splints for the past month or so. Shes a field hockey player. How should we treat it?

Rest. Shin splints occur when the muscles of the lower leg develop too rapidly, causing the compartment to stretch and put pressure on the nerves in the leg. Treatment is to rest until the pain stops completely, then gradually work back up, allowing time for the compartments to grow with the muscles. When you do begin working again, stretch and ice frequently.
Gradual return. Shin splints occur when the muscles grow to big, too fast. Usually this is from a rapid increase in activity such as training or starting a sport. The best way to treat these is rest, ice, and stretching...Followed by a gradual (slow) return to activity. Even after working back up, ice the legs after exercise, and stretch before and after exercise.
Female triad. Checkvitamin d level. Have md screen for female triad common in female athletes (low energy availability, irregular period/low estrogen, osteopenia/porosis). Make sure her nutrition matches her energy expenditure, vit d and calcium intake is appropriate , have proper rest period.