Sounds like an over-reaction to me!
Sure we can all argue that caffeine should be avoided, all things in moderation, etc. but it is not completely unreasonable for something with caffeine to be offered to a 15 year old.
And a once in awhile offer should not cause heart problems or addictions...at least in almost all cases.
It is also not *completely* unreasonable for the parents to prefer their child to not have caffeine. But, if they wish for their child to avoid caffeine, they need to express those wishes to anyone who is given an opportunity to be responsible for her. A simple, will you be serving anything with caffeine or sugar? would do. And then the parent could choose whether or not their DD or DS should attend or BYO or whatever.
Of course being overly protective can have draw backs down the road (the rebellions mentioned...). Not being protective enough has other draw backs, which most of us are also aware of....balance, balance, balance....
Honestly, whatever parents wish to protect their children from, they should protect their children from...(Me, I make sure there isn't any guns where my children play - must admit I've been a bit lax on checking the premises for caffeine

even though we do avoid it at home). What kind of parents would we be if we didn't protect our children what we believe (rightly or wrongly) is dangerous!?
As for the teen telling her parents, sounds like an unstable household. IF this is the environment in which she was raised for 15 years, how can you judge or blame her for her actions?! SHe's lived with kooky people for 15 years -- she may do some kooky things herself.
My statement about the parents over reacting isn't so much about whether or not caffeine is ok - it's about the fact that they could have and should have inquired in advance if it's something they feel strongly about and it's about that fact that they could use this as a learning opportunity with their DD to reinforce their believes, rather than throw a fit over it.
Oh well. Sounded like a fun sleepover to me. My DD5 loves to bake and I'm sure she would do it at all hours if I let her (she is a tad bit young to bake all through the night, but I sometimes do!) And my DD 3 loves coffee. I mostly drink decaf so I share. When I do have regular, I don't share...so she asks, in her 3 year old voice, "Is it decaf?" Too cute.
Hope my DDs have fun get togethers like this at my home with their friends when they are older!