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In order to transfer a system, they need to be at peace, and have a claim on the system you want to give them, or having a system adjacent to the system you want to give them.
If they're friendly with you and currently being actively destroyed, that seems unlikely. (unless they're being attacked by the crisis). If it was a crisis attacking them, you could tag switch to their country, make the claim, and tag back to your country, and give them the system. (Providing you're not playing iron man).
