Given the option between his mom sending it unencrypted, and encrypting it with an easy-to-use extension he set up for her, why not encrypt it? Surely it can't _hurt_ an already horrible situation?
Yes, in this case, bad crypto can make a bad situation worse: it can create a false sense of security.
And in the general case, bad crypto does much worse things than that. Logging in without a password. Flipping bits in a cookie to become any user on the system. Gaining admin privileges. Spoofing authorities to collect secrets from users.
Bad crypto does not care how horrible your mom has it. Engineering says, bad crypto is going to be bad no matter how hurt your feelings are.
Moreover, for noncritical things, like my example of fantasy football plans, this appears to be a perfectly reasonable solution.