Pictures on iphone not getting received by others?

I have an iPhone and I can send pictures to my mom sometimes, who also has an iPhone. They don't always go through to her, but most of the time they do.
I'm also trying to send pictures to someone in Vegas (I'm in Toronto, Canada), and he's not receiving them. I sent three of them, one right after the other, but he didn't get any. I sent them over text message, and it looks like they sent normally on my end, but for some reason he's not getting them.

What setting should I change my phone to or what should my friend change his to?
Thanks!

(Also, he has an iphone, but when I try to send the pictures or message him, it's coming up as text messages, not imessages.)

Added (1). And he's just visiting vegas, he live in toronto like me.
Maybe this has to do with it not working?

Well iPhone have some security issue that's why you have problem in sending pics better you can send your pics from some messenger apps like whatsapp, Viber etc.

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