In my experience, the majority of girls have no idea what the cost of a dress alteration will be. They have a preconceived notion that all the work they need to have done will only be a few extra dollars. Sometimes when purchasing a gown, the salesman will lead you to believe that an alteration will be cheap and easy. This advice either comes from an inexperienced employee or a dishonest one.

Dress alterations are not the same as shirt/pant and other dress related alterations. If you know someone that sews, great, but that does not qualify them for bridal alterations. Many bridal boutiques have qualified seamstresses or will recommend someone. Usually, if they recommend someone, it is because they have done work for them before and their clients were happy with it. I would suggest going with a recommended seamstress from a bridal boutique over a friend.

Keep in mind, you will not be their only client, and they are likely managing multiple time frames. Agree on a date for the gown to be completed, especially if you want it sooner than the actual event.
My first job in the bridal industry came in the alteration department. I saw first hand the frustrations of altering wedding gowns and the time and work that go into them.