I drink hibiscus tea for the same reason., although I did not make any measurement of before and after difference. It is recommended 3 cups a day, and I drink less than that.
I make a rather large pot, let it cool in the fridge and use it as my daily water intake with a bit of lemon. It's really refreshing. I buy the tea from the health food store, in teabags, and I also believe the loose petals can be purchased.
Glad to hear your relative is looking to improve diet and exercise.