I couldn't think of any way that the letter could have happened without the show contacting someone ahead of time, but it's a lot of work for them. Depending on how someone buys the ticket, the show would potentially know who bought the ticket and how to get in touch with them. Do you know how your mom bought the ticket? Also, did your mom's husband's letter get used? I imagine they would have to have a backup in case someone doesn't show up.
In terms of the letter picking, I wonder if they xerox the letter and every envelope has the same letter in it.