Maybe just say, don't ever give him coke again. He's a baby & does not need caffeine, sugar & all the other crap in it! How old is she? My 19 year old sister has always stuck by my rules even though she was only 12 when DD1 was born. So have my 16 year old SIL & 13 year old BIL.
Unfortunately, she's 36. With a 14 year old of her own. You would really hope she knew better, but no