There are more articles about this out there.
Can I imessage someone when I am in another country if I don't have international calling package?
......If it is supported in the country you are traveling to, but you are not required to have a special feature on your account, unlike making voice calls. Keep in mind that unless you have a special International messaging feature, you will be charged a higher fee to send messages while roaming internationally.
Someone wouldn't be charged anything additional for sending an iMessage, only SMS. As long as they are only connected via wifi, and have turned off cellular data, they could use it nearly everywhere, with the exceptions of ISPs of certain countries that 'fair use' applies, allowing them to block some or all of Apple's services.
Having Wi-Fi troubles?
From the garage to the basement, we blanketed every square inch of the CNET Smart Home with fast, reliable Wi-Fi.