That's a toughie! I -sort- of can relate to the embarrassment of memory loss, not from a dissociative disorder but rather from ECT treatments.

You were good friends with this person, did she seem the type to be supportive of people with mental health issues? If so, she could be very helpful in filling the gap of memories that you had lost. If she is an understanding person, she should not be insulted that you don't remember her wedding because it wasn't your fault in any way.

You don't even need to go into details on the cause of your memory loss if you don't want, but rather a vague "I struggle with memory loss and had some questions I was hoping you could help me with" kind of thing.