As a professional seamstress who designs and makes wedding gowns and who used to work in a bridal shop doing alterations...........
I will tell you that knowing you just had a baby, I would not even take you in for a fitting at this point because your body will change, for absolute certainty. Two weeks after a baby is born is not the time for a fitting, not even close, since the wedding is 8 months away. I would want to see you 6 weeks before the wedding if I were doing your alterations. Maybe 8 weeks prior if you were not local.
Yes, a fitting means the seamstress is pinning to alter the dress as pinned. If you come back after the fitting and the alterations and you have lost more weight, prompting another fitting and pinning, that means there will be another alteration, and an additional expense for having to do the work twice. Don't do that.
I do understand that the bride is anxious and wants to check things off of her list, but this is unreasonable to have a fitting two weeks when you have so much more time than that to get this dress fitted correctly the first time.
If it is an issue of location and travel time to a city or state far away, have the dress shipped to you and have an outside qualified seamstress do the alterations in your town. A private seamstress will charge you far less to alter a dress compared to a bridal shop as their prices are super duper inflated, I know that for sure.
I hope this has been helpful and will set your mind at ease. Think about what you want to do and then talk to the bride to calm her fears. She may just be anxious that you will have no time once the baby is older. If you do decide to use a local seamstress, find one before you call her to discuss this matter. If you have all of the answers, she will have a sigh of relief, I assure you that.
EDIT: My above answer was based on if your dress was already at the bridal shop and needed to be fitted to you. If you mean to ORDER the dress, two weeks after the baby is born, then go ahead and get measured and insist they order you a dress a size smaller -- which can be altered when it comes in and yes, you will have to pay for any and all alterations. There is no charge to get measured to order a dress as this is how they know which size to order.
If you live in a distant city, a bridal shop in your city will measure you and forward those measurements to your friend's bridal shop and there will be a charge for that. You need to figure out if the travel time with your baby, the gas and all that is worth the trip or if it is easier to get measured locally. Hope this covers it all.