Not sure if he has a recent iPhone with eSIM, but there's a nifty little feature to keep in mind that can be helpful. In the case of dual SIMs, iPhones can pipe voice calls and texts over the other SIM - it's automatic and uses the same mechanism as Wi-Fi calling. As long as you have Wi-Fi calling turned on, and you're not in a country that AT&T/Cricket Wi-Fi calling doesn't work in (China, Saudi Arabia, India, Turkey, Israel, United Arab Emirates, Vietnam, Pakistan, Cuba, Iran, North Korea, Sudan, and Syria), your Cricket line will say "Cricket using Cellular Data" and use data on the other line (Airalo, local SIM, etc.) to work. Folks without iPhones sometimes don't believe this is a thing, but it's basically just creating the VPN tunnel for Wi-Fi calling over a different data connection - I use it domestically with my Cricket line and a T-Mobile line as backup (for the rare cases where one works and the other doesn't).
As Cricket doesn't have international roaming outside of North America, it would fall to "No Service" the minute you're outside of the US/Canada/Mexico. At that point, the phone will try to establish the tunnel either over Wi-Fi or a functioning data SIM.
https://support.apple.com/en-us/HT209044 (scroll down to "Learn about Dual SIM status icons")
I had a family member who went to the Netherlands last month. Cricket + Airalo on a 2020 iPhone SE was a really good combination for her. Other than keeping an eye on data usage, everything worked as expected and she used normal voice calls and SMS quite a bit using the method above.
Additionally, by keeping the Cricket SIM active (as opposed to removing it for a local SIM), it keeps iMessage and FaceTime registered, and that will use whatever the "easiest" data pathway is (Wi-Fi, data-only travel SIM, etc.) before falling back to SMS.
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