is it a bad habit to give LO a snack before bed
She is wrong, dead wrong. Breastfeeding is an on-demand system; you feed your baby (especially a three-month-old infant!) when they are hungry, whenever that happens to be. This keeps your supply up and the baby gets just the exact right number of calories that she needs for the day.
Why on earth would it be unhealthy?!
Second, you aren't forming ANY habits with a 3 month old anyway, you can't, too little!