The sad thing here is that nobody is pointing out the most absurdly ahistoric thing in this scenario, which is that a European nation was actually bordering Navajo in the early 1600's.
Why complain when a nation tries to modernize? It's not out of the realm of possibility that they'd have done that, had history gone differently. If you're OK with Europe being there at 1627, calling out the early westernization is nonsense lol.
Even in 1.5, it's possible to reach North America and have a huge established colony by the early 1500's (IE barely after 1500 at all).
Yeah, I just vassalized them. I will check tonight for more details on dates. It seems that they started westernization before Castille had a core - if not, then they did it in breakneck speed. It would appear that they saved up their Monarch Points and Gold for the entire game - essentially doing nothing until - Westernization.
Nations will appear western the *instant* they click the button, despite not being finished.
Also, with the reform mechanic, Navajo had to have had at least some time prior to westernization. You can't westernize before reforming, and after doing so you're only 6 techs behind.