The bride-to-be is not at fault here. It is her engagement party, and she has the right to celebrate it when most of her friends can attend. The waitress friend's schedule is simply incompatible with the rest of the group, and it's unreasonable to expect everyone else to accommodate her constantly. It is her special day, after all!
Hold on, let's not pretend the bride is completely innocent. While it's her engagement, she's showing a distinct lack of empathy. The waitress friend is clearly feeling excluded, and a little flexibility could go a long way. The bride should at least consider rotating event schedules so everyone can participate sometimes. Her actions speak volumes about her priorities, and they are not inclusive.
While it is understandable that the bride wants to celebrate her engagement with as many friends as possible, she must also recognize the impact her decision has on her friend. The waitress friend's feelings of exclusion are valid, and the bride could have shown more empathy by at least exploring alternative solutions. However, the waitress is not exempt from blame, as she needs to understand that everyone has different schedules, and it is up to her to plan around the group's schedule rather than the other way around.