How d o you give constructive code feedback
Just telling someone that their code sucks, is not gonna improve their code.
If you want to tell someone that their code is bad, explain why, provide examples how to do it better and then explain why your examples are better.
If you cannot provide that, then don't give feedback at all.
Expect the other person to be a beginner, expect them to ask you questions why exactly their code is bad, and be ready to answer those the best you can.
Be friendly, telling someone that their code is absolute dogshit is just gonna make them feel bad, and won't exactly make them eager to improve it, but rather,
close the chat window or go away. Instead say something like "This is bad, it is bad because ... and here is how you could improve it"