iMessage undelivered to a specific contact on iPhone

My 80 year old mother-in-law called me yesterday to inform me that when she messages a particular friend the message gets bounced back as ‘undelivered’… This only happens with this particular person as other friends all receive her messages. The person in question can send her messages which she can read but not the other way around. The same person informed her that she has not been ‘blocked’ on their phone and all other forms of messaging work, What’s App etc.

Would you have any suggestions for me to assist her please?

Thanks.

[Re-Titled by Moderator]

Original Title: Undelivered iMessage