Is she not drinking anything else? The urine being concentrated could indicate she isn't getting enough water or there could be a UTI or kidney infection. Is she in pain? If she has any lower back pain it could indicate an issue with her kidneys and to see a doctor.
If it's just constipation, I would say it isn't the green tea, but dehydration in general can cause that too. Water - not just tea, water is imperative. I would tell her to flush her system with pure water for a day and see what happens. Perhaps drink a single cup of senna leaf tea to get things moving again to relieve the constipation.
Fiber is good, but don't increase fiber until after you've got things going again or it can bind you up worse.