You cannot send a single message as both SMS and iMessage. It will be sent to either one network or the other, based on what address you use for sending (you choose between eligible addresses when you initiate a new message - if the recipient's contact card contains both phone number and Apple ID email address, Apple Contacts will color-code based on the recipient's situation - blue for iMessage, green for SMS. So if the phone number is green and the email address is blue and you want to reach the person by SMS, choose the (green) phone number.
However, if the recipient's phone number and email address are both blue, this indicates the recipient's phone number is registered with iMessage - it will try to send iMessage to the phone number, not SMS. If you have "Send as SMS" selected, it will send the message as SMS only if that number is not reachable by iMessage. In this particular case, if the iMessage sent to the phone number is received by the iPad, then by definition the recipient is reachable by iMessage and no SMS will be sent.
So yes, the recipient turning off their phone number in iPad > Settings > Messages > Send & Receive will make the recipient's phone number "unreachable" by iMessage and Send as SMS will then kick in.