Cow milk? If I'm warming milk, I do it in the microwave. In a separate container, usually a mug, stir and pour into the bottle/sippy. Never had a problem doing this, always testing before giving it. I don't think there is anything wrong with warming at 12 months, and then you can slowly wean the child onto cold milk yourself over the next couple of months.

Just out of curiosity, do you know why is the parent insisting? Is it a digestion issue or just a "we do it at home, so you have too also," kind of thing?