I guess everybody has a different way of doing things. Personally I'd ask my colleague on Slack (privately), "Hey, you repeat 10 lines of math in each of these functions, wouldn't it make sense to make it a separate function?".

The two probable answers are: "No, because... (reason)" or, less likely, "You're right, I'll do it tomorrow". No fuss, no problems for anyone, and someone can learn a thing or two.