My friend says its gramatically incorrect, what do you guys think?
For informal speech, this phrase is okay. For formal speech, or for something written, such as a persuasive letter or a term paper, it would be better to avoid combining “mind” and “mindset” in close proximity. The awkwardness would be kind of like writing, “Next, set the new combination on the combination lock.”
Oxford’s definition of mindset:
The established set of attitudes held by someone.
To be in the right mindset means to have a helpful mental attitude. For example: if you go to college thinking it’s good enough to show up to class and spend an hour a day on homework, you’re not in the right mindset. Meaning, you aren’t looking at the college experience the right way — you’re not thinking about it right.
The simple correction to “he has his mind in the right mindset” would be “he has the right mindset.”