Turn off "Send as SMS" for once recipient

No, you can turn off send as SMS for everyone, or leave it on. But it's not selective at all.

I assume she's sticking to free wifi only then while traveling? If so, maybe just use email only while she's away? That way, you can send your message whenever you wish, and she'll get them and can respond whenever she has access to wifi.

You could try sending the messages directly to her iCloud email address.

As long as its not already saved under her contact, this may give you the opportunity to start a new, separate convo that is only possible to iMessage.

I used to use this trick with friends who are overseas to prevent accidental sms charges for international numbers and such.

You can also long press on any message while it is sending and simply choose the option that says "send as text message." This option is only accessible when the message is still sending. (So when the progress bar is going across the screen at the top." This option is especially helpful when the recipient cannot receive imessages.

The menu should look something like this:

https://i.gyazo.com/c802f5360ada114588162fd1838ceaea.png

Edit: Sorry, I missunderstood this post.

Sorry, your reply to my reply collided at the same time of posting 😁 I'd suggest just using email while she's away.