First, there was not an account involved, so whether the son was "on the account" is irrelevant.
I'm going to argue for including the son as a person on whose behalf she completed the cashier's check purchase. She is the purchaser, without a doubt, but she made the purchase on her son's behalf (to pay off one of his obligations). So I would include a part 1 for Mom with 2b checked, and a part 1 for Son, with 2c checked.
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8