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I have a co-worker who I got to know through a mutual friend before we started working for the same company, although he s been working in the company for a year more than me, he was kinda relaxed and didn't take much responsibility and doesn't have a good reputation among his seniors,
I recently joined and worked hard to understand the product better from a technical standpoint and have a better hand at it now.

My colleague recently started developing a feature for the product and is almost set to completion, he now asks me to review his code before going through the formal code-review process, I can see a lot of flaws in his code already and I want him to learn, at the same time I strongly feel that I want to review his code formally just so that people know that I have helped him in coming up with a better code than what he already has, another reason is because I want him to work hard and not have everything spoon-fed to him.

Am I being too selfish here? How should I react to this ?

I can see a lot of flaws in his code already and I want him to learn,
at the same time I strongly feel that I want to review his code
formally just so that people know that I have helped him in coming up
with a better code than what he already has, another reason is because
I want him to work hard and not have everything spoon-fed to him.

OK, there's a lot going on in that statement. Let me break it apart.

I can see a lot of flaws in his code already and I want him to learn

Are these flaws quantifiable, meaning would any other developer also see them as flaws, or is this your opinion? Are you being subjective or are you being objective in your analysis?

at the same time I strongly feel that I want to review his code
formally just so that people know that I have helped him in coming up
with a better code than what he already has

Is your goal to help your friend or is your goal to get some kind of credit for yourself for helping your friend? What's your true motivation?

another reason is because I want him to work hard and not have
everything spoon-fed to him

That's a bit condescending. How do you know that this person doesn't work hard? Are you the arbiter of who works hard and who doesn't? Do you make the determination of what level of effort constitutes hard work?

Is asking you to review his code analogous to being spoon-fed? Is he asking you to review it or to correct it? Is he asking you to do his work and research for him? If not, then he's not being spoon-fed.

It all pivots around the question if your friend has the professional authority to ask you for this. If not (which i have the impression of) then it is a personal favor, i.e. off the clock, and off the record. Do not let the work assigned to you by your boss rest because of this task.