"I'm loving isolation. A little too much."

Is the following statement grammatically correct?

"I'm loving isolation. A little too much."

I think the statement is grammatically incorrect, but someone, a professional writer/editor, made that comment on Facebook, and after I called them out on it, insists that it is correct.

I see that it is trying to replicate natural speech, but the way to introduce a pause would be with a dash or an ellipses, not a full stop.

So which one is right, a period or something else?

"A little too much." is a sentence fragment. It is not a complete sentence because it does not contain a subject and verb.

However.

This is a stylistic choice. It's playing with punctuation. It may not be acceptable in formal grammar to use sentence fragments. However, writers are within their rights to use words in whatever way they desire to get their point across to readers. Novelists employ many techniques and structures that may not pass formal grammar rules -- but that does not make them wrong. I think this is effective, interesting writing. There are more important things to argue about.