I would not offer formula. Most likely if she is not used to it she will refuse it when it is offered at her dad's. I would provide pumped milk. You should probably experiment with different bottles or sippies. At 6 months babies can have water so put water in a sippy cup for her to play with and experiment with. I would type out a list of what she can eat (only bm, a little water-usually no more than 4 oz and whatever solids she has had) and then have the doctor sign it. Give a copy of that to her dad and tell him that this is the feeding schedule approved by her doctor for her age. Most daddies want whats best for their little one too. Take comfort in that.