Are you using any 3rd party app for messaging? Are you on mobile data or WiFi Calling?
Tried both. Maybe it is wifi but I've sent text images many times before on my wifi. When I'm at home wifi calling is always on. Been that way last couple years. It's just this one person though I wonder if it's their phone.
Have you tried looking in the stock messaging app to see if you have any 'missing' texts there? It might be the RCS or 'Enhanced Messaging'. Whenever I have that enabled, 3rd party apps that can't access RCS (or so they claim) ended up missing texts and MMS from other RCS-enabled users. I turned that off and the issue went away. T-Mobile, unfortunately, doesn't let you disable it anymore so I have to work around it with my contacts. I don't think you can turn it off on the S8, but maybe your friend can turn it off on theirs.
So I sent myself a text message with an image in it and it worked for me. Must be a problem on this persons end. I think they have a One Plus 5 currently. This guy owns like 4 phones but they're low to midrange at best. He likes to buy stuff like the j7 and that type of stuff that's cheap and then he always dislikes something about them. I told him buy a quality phone like the s8 Plus or something.