How to iMessage a different country?

If you see a green message bubble

If you see a green message bubble instead of a blue one, then that message was sent using MMS/SMS instead of iMessage. There are several reasons for this:

• The person that you sent the message to doesn't have an Apple device.
• iMessage is turned off on your device or on your recipient’s device.
• To check if iMessage is turned on for your device, go to Settings > Messages > iMessage.
• iMessage is temporarily unavailable on your device or on your recipient’s device.

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

About iMessage and SMS/MMS - Apple Support

Set up iMessage and SMS/MMS on iPhone

Use Messages on your iPhone, iPad, or iPod touch - Apple Support

Talk to your friend, he may not have activated iMessage on his iPhone with the Phone number Either

1. Send him a text on his email ID registered with Apple ID
2. Ask him to activate his iMessage on his iPhone

If what you are seeing is green, then that is not an iMessage, that is a standard text message SMS and would be using the cellular system and would more than likely be considered an international SMS, which is probably why it is not going through. iMessage would require that both of you have iMessage activated, you both must have an active internet connection, either cellular data or wi-fi. Why it doesn't work could be any combination of failures. But everything listed about must be present for iMessage to work.

That means she either has imessage turned off, or is in an area with a weak wifi or cellular data signal.

Make sure both of you are provosioned by your phone carrier to use SMS.