I guess this may be the wrong place to ask about negative effects from coffee... I am sure the average daily consumption of caffeine here of members is well above the National average.
Really there have always been two distinct schools of thought on coffee consumption- and these are pretty much polarised. Scientific reports (often underwritten by associations such as the NRA) have shown coffee can prevent Alzheimers, help prevent bowel cancer etc. Other reports (underwritten by coffee opponents) show that coffee can accelerate heart disease and adversely affect kidney function... no wonder so many people have questions and concerns like you have.
Really coffee consumption should be enjoyed "in moderation". I would say dependng on the body mass of the individual, a max of 2-5 cups aday is OKfor most people. Gallons, certainly is extreme. I would watch out for your daughter displaying habits that are associated with excessive caffeine use (abuse)- irritation, insomnia, jiggling knee syndrome, over active...etc. This in itself may not be definatively a sign of drinking too much coffee (teenagers often display at least 1 or 2 of the symptoms listed anyway, regardless of whether they drink coffee or not!!), however it is a guide to over use. Maybe point your daughter towards drinking decafe? Maybe ask her why she likes coffee...it could be she likes the milk, in which case many cafes serve just steamed milk, without coffee...
All in all I would not be too concerned. When I was a teenager I was drinking 17 cups a day. Granted, it led to my life in the coffee industry, but many of my friends who also drank a lot are now leading normal lives, drinking 2 cups aday and holding down good jobs in offices (while I rove the wilds of Indonesia!)