My MIL was a paediatric dentist. Cue vigorous brushing when we go to Granny's house .
She badgered us incessently about DD having bottles and a dummy. Apparently milk contains a lot of natural sugars (even breast milk). Funnily enough, our dentist was pretty cool about it, and said so long as it didn't continue beyond 3yrs, it wouldn't be much of an issue. Having said that, I did take my MIL's advice, and watered down my DD's bottle when she was old enough to be getting enough calcium from the rest of her diet.
I have read some literature lately about teeth brushing, and it seems that enamel erosion is as much of an issue as decay is. It seems that brushing teeth too soon after eating / drinking can cause erosion. IIRC you should wait at least 1/2 hour after food / drink before you brush.
According to my MIL, although baby teeth are going to be replaced anyway, if you lose your baby teeth too early, then it causes problems for the adult teeth coming through, as the jaw isn't developed enough, and it will cause overcrowding and the need for orthodontics in later life.
TBH you 'doctor' friend sounds quite overbearing and bossy. I think there will always be an inevitable balancing act between competing needs - dental health versus the need to get enough calcium in the diet versus emotional / behavioural development - I could go on, but hopefully you get my point.