How do you work with colleagues who write messy code?

Context: I work in a small start up, 3 people. Two of us are dev's. I myself try and strive to make my code readable and maintainable, to explain my self as often as possible and to help out the other dev with learning things that help us communicate (git etc).

The other dev is fantastic at getting things done asap, which is great for shipping but as soon as things get a little too complicated and he is struggling with his code base, he attempts to off load the problem onto me to solve (deferring to my experience).

I should note that, though we are equal part in founding the company, he has taken on more of a technical lead role.

The Problem: I keep being passed these problems to solve that should be simple but because of his bad coding practices (copy and paste code all the time, ignore failing tests, premature optimisations etc) it takes me a long time to:

A: grok his code (making me look bad because i'm mean't to be the experienced one)

B: make non-breaking changes (there are no tests, the code base is very messy, both in abstractions and format)

Any advice on how to tackle this? Perhaps approaching a conversation to make him aware in a way that won't burn any bridges?