Try separating out the factual observations from how you feel about them. "The main logic is in a single ~400 LOC function" is a simple observation and less likely to make your co-worker feel attacked. I think the word "monolithic" is counter-productive.
"I noticed that the main logic is in a single ~400 LOC function. I'm concerned because I may need to make updates to this in the future, and larger chunks of code are harder to work with. Would you be willing to pull it apart into smaller functions?"
I think that "I wouldn't dare touch it with a 10-foot pole" is something that I'd leave out of the PR comment. The observation that it is complicated, and the concern that future work on it would be difficult, that can definitely be included.
"I noticed that the main logic is in a single ~400 LOC function. I'm concerned because I may need to make updates to this in the future, and larger chunks of code are harder to work with. Would you be willing to pull it apart into smaller functions?"
I think that "I wouldn't dare touch it with a 10-foot pole" is something that I'd leave out of the PR comment. The observation that it is complicated, and the concern that future work on it would be difficult, that can definitely be included.