What can a student do if a professor makes special allowances for one student in a course?
The arrangements that the professor makes with any other student are absolutely none of your business. I don't see any indication that your grade is threatened. If you are learning what the course aims to teach, it is not your place to second-guess the way that your professor chooses to run her class. 

Faculty are often asked to make accommodations for individual students for any number of reasons, and these reasons are not to be made public. In fact, I'm required by law both to offer alternative forms of evaluation where appropriate, and not to explain why I am offering the student an alternative form of evaluation.

For a US perspective, read up on the ADA and FERPA and you'll see an interesting intersection of required accommodations and required privacy on behalf of students with special needs.

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