My friend's iphone keeps forgetting one of its iMessage addresses. Both the phone number and her email address are configured for iMessage but often the phone doesn't receive messages if they are sent to the email address. This often happens if she sends me message from the Messages app on her mac and I reply on my mac. Then she expects a reply on her phone but doesn't get it. Is there a way I can configure my Messages app to always send to the phone number? I guess I could just delete her email address from her Contacts card but I don't really want to do that and it might not work anyway.
Tell your friend to go to Settings > Messages > Send & 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.