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Do they happen to use the Russian App Store? If so, the app hadn't been updated to utilize the new protocol because Apple had delisted[1] ProtonVPN in mid July.

[1] https://apps.apple.com/ru/app/proton-vpn-fast-secure/id14370...

He is permanent resident of Spain but he is in Russia currently. So he has Spanish AppStore

The issue reported here (unanswered since March) says they are using Stealth in Russia and it is still not working: https://github.com/ProtonVPN/android-app/issues/130

This your friend? https://github.com/ProtonVPN/android-app/issues/130

ProtonVPN's IP ranges blocked?

I dunno. But they advertise their "Stealth" protocol as a solution for everything. And there are still problems.