Any nation using nukes and getting away with it would trigger a massive nuclear race in the entire world as everybody tried to nukes (either to defend themselves from those with nukes since the possibility of nukes being used offensivelly had become a reality, or to themselves engage in wars of conquest using nukes against nations without nukes) and in a World with many more nuclear armed nations sooner or later other nations too would use nukes, in an escalating fashion, ultimatelly leading to nuclear war all over the planet and the end of our civilization as we know it and possibly even Humanity.

This is why EVERYBODY, allies included, would turn against any nation that used nukes - it would be the only way to stop an escalation that would innevitably lead to a nuclear holocaust.

For all their threats to do so, it’s not by chance that Russia did not use nukes in Ukraine: amongst others the Chinese made it very clear that any use of nuclear weapons was a hard red line for them.