A friend of mine had failed core courses during his master's program. He had to retake the exams and passed them with minimum grade. However, his research output was good and he was able to publish a journal paper (first author, IF~4).
Now he has been accepted for a PhD in Canada on the same topic as his master's. He will be working with application of the courses he had failed in.
Should he continue with his PhD journey? At the university he will be carrying out his research, they don't have to take any coursework. So, he would have to continue with his research without having to take any courses.
Is he suitable for PhD? As far I know, PhD students are supposed to be proficient in the coursework.