It is trivial to find more than sufficiently authoritative source that cover the rules that make "Chris'" and "boss'" perfectly value contracted possessives in English. [1]
However, it's English: there isn't just one rule, another rule can also be valid and might be the one you're familiar with on a day to day basis. That doesn't mean any other way to say or write the same thing is wrong, it's just a pattern you never saw. Like someone going "lol snuck isn't a real word, it's sneaked!" and then you hand them a dictionary and they learn something new about their own language.
However, it's English: there isn't just one rule, another rule can also be valid and might be the one you're familiar with on a day to day basis. That doesn't mean any other way to say or write the same thing is wrong, it's just a pattern you never saw. Like someone going "lol snuck isn't a real word, it's sneaked!" and then you hand them a dictionary and they learn something new about their own language.
[1] https://www.ox.ac.uk/sites/files/oxford/media_wysiwyg/Univer...