How to resolve the ambiguity of sending iMessages to a person with multiple addresses?
Tell your friend to go to Settings > Messages > Send &amp; Receive.

By turning on the setting to receive from all of the email addresses, it shouldn't matter which address you send to. Your friend can also choose the phone to be their "Caller-ID" so they are sending as a mobile and not an email/AppleID.

You can also be sure to delete any conversations that aren't to the phone number so you reuse a thread to the cell as opposed to the email.