We are a small team of developers working on an IT project within a dedicated department. No hierarchy between us in that department, but one common goal: to get our main application up and running. We all work on the same code.
I am quite detail-oriented and take a great care when it comes to standards, clean-code and architectural concerns overall.
Now, one of the more senior devs happen to patch the code in a way that produces obvious bugs. Also, his code changes do not respect a certain quality standard. Basically, he introduces lots of bugs due to a care-free attitude, or at least a reason I cannot fathom at the moment.
The Department Manager does not care for cleanliness and operative details. He wants the main app to run and us to be able to add new features to it. He implicitly want us to take care of those details. I do think that quality standards and code maintainability is important, he has accepted that fact, but nothing is formalized in that sense. Pretty much "do as you must, it has to work I don't care how" territory.
I do not have the authority in terms of hierarchy to 'correct' the colleague. However, since he produces bugs, I want by mere friendly influence get him to avoid those bugs. I happen to hide those bugs from the manager and try to resolve them with said colleague first. He seems to be good willing but does not look like he learns. I have also in the past had verbal altercations with him since he thought I was meddling too much in "his affairs". But mostly, we get on quite friendly and have mutual respect. I might be perceived as a holier-than-thou guy, which does not help. I am nonetheless taking that mantle though, feeling the need for it.
How do you proceed to politely but efficiently help one such colleague to improve our code quality? This is mostly a communication question.