Dear coworker,

We have a very nice SASS setup so WHY THE FUCK ARE YOU POLLUTING OUR HTML WITH INLINE STYLES!?!?!? It's not like it's even necessary to use inline in this case, I can achieve the same thing by just moving your styling to the scss files. So I ask again, WHY THE FUCK???

Sincerely,
A mildly pissed person who hates front-end dev work

Inline styles?! My eyebrow is now twitching. M

Speak politely to your colleague. Find his experience with CSS preprocessors. If he's a novice then suggest some resources to self-study. I have interviewed at least 20 frontend developers in the past and most of them surprisingly didn't practice it well.
Don't you have a way of working or best practices defined somewhere? Perhaps, you can share this documentation to your colleague. I believe you do have code reviews. Write your comments there so he may fix his mistakes and improve.

Does he even know...? Then again, even then it shouldn't be inlined.

That's why I'd do code quality tests before merging..