only water and peace

You can't stand on your head enough different ways to make people happy. Don't drink anything else other than water. That person needs to read about fasting - give the information and decline politely. If they knew you give away too much of your own comfort for their perceived comfort, they shouldn't be asking you to do whatever.
Or avoid people and their input altogether. Fasting isn't good time for socializing, except if someone knows what it is all about or if they are fasting as well. It is by no accident that ppl resort to fasting retreats or isolate themselves or go to the desert - in the case of Jesus :)
Ensure you have quiet and peace.
Blessings,
DL