Possessive determiner followed by a stand-alone adjective?

My grammar lecturer told us that "I thought her selfish." is correct and it is used like the sentence "I thought she was selfish." He said native English speakers use this structure a lot, but us non-native speakers don’t. Which confused me.

I asked my friends who were native English speakers and one of them said that it’s old English. I tried researching about it but to no avail. Can someone please justify why "I thought her selfish." is correct grammatically, even if it’s an outdated grammar rule? I really want to know.