Root [International] Adding emoji converts it to MMS?

[MrMister]

Sister just got a S4 in UK. I only dabble in Android as have a fruity phone

Anyway she had a old Nokia before and used the keypad to put in emojis on her SMS messages. She very un-tech and so I normally get asked.

She started SMS, added an emoji on the keyboard and it flashes up a message "converting to MMS". I can't find a way to stop this. Obviously MMS cost loads, whereas before they went in message fine on Nokia. She's on the same network etc.

So I looked in settings but couldn't find anything really. Can anyone help please, she's lost without her emojis!!

Thanks