Re: Antibiotics and venting sesh...
I would call the doc and ask about whether or not the abx should be given with food. The presence of food in the baby's tummy can alter the rate at which the antibiotics are released into the bloodstream, and breastmilk digests so fast that the antibiotics might enter the bloodstream too fast, or in too high an amount? IDK, it's probably fine. But I would still call and check!
I would definitely tell your friend what her mom is up to with the formula. If it were your baby, wouldn't you want to know? And I know it can be hard to get involved in a touchy situation involving an aggressive person, but just imagine how your friend might feel if someday she finds out that not only was her mom doing something underhanded, but that you knew and you didn't tell. In the end, you want to maintain your friendship with your actual friend, instead of living in fear of her mean mom.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"