The "Professional Computers person" probably chose not to include screenshots because it would debunk his feigned complexity.
Since he's already claimed that his mother won't receive the messages•, - then the whole faux-drama would have been solved by simply turning off iMessage on his or his mother's device.
(• Which is false: the messages would be sent to her mac, and they'd be visible as unread to him, so he'd already see that there was an issue.)
It's also not some obscure hidden setting to choose a different delivery pathway in the Messages app: it's one button and the -only- option on the screen when choosing a recipient. It's how a normal, not-tech savvy, person chooses a different phone number for a contact that has multiple phone numbers, and iMessage will remember that choice for the future.
And if all of that is too hard, his mother could simply send him a message and the messages app will reply to her phone number, it won't auto switch to her email address. When he composes messages in the future, the Messages app will also put up that SMS pathway as the 1st choice.
This article was written to manipulate people into believing a false narrative, and judging from the comments, it worked great.