Huh, that's a good point. A better universal naming system would be something like "Base x+1", with x being one integer lower than 10. So humans would use Base 9+1, and the alien would use Base 3+1.

*This has been on my mind all day and the more I think about it, the more obvious it becomes how fundamentally terrible the name "Base-10" is. How did this never occur to the people who coined the term? Even the system I suggested is flawed as it's still trying to incorporate the same bad logic.

A better system would be something like Base 9, stopping shy of the respective 10 in each system, or if it needs to be clarified, Base 9+0, as 0 is the extra digit in the first place, not 10.