Math tutor is evading or giving wrong answers
I don't know about German technical colleges, but in the US and in Switzerland, students do have a chance to give feedback about the teaching. This sort of information is very useful then! It has the benefit of usually being anonymous. But when you do give this information, make sure that it is factual and detailed (so something like what you wrote in the question) so that the professor has a point of entry for the investigation. 

If I have an assistant who behaves exactly as you described, I would be very unhappy with the assistant and will be thankful for any student who brings it to my attention. 

Now, to answer your specific questions:


How to tell the professor

a. Again, I don't know about the German system. But if you don't want to be confrontational there are often options of bringing this up through the student union or the association of mathematics/statistics students. In some universities there are also student advocates who can act as intermediary for you.

You also mentioned that other "better students" don't know about this. Perhaps you should drag some of your friends to the office hours and show them what is going on. You will get a sanity check on whether your impression is correct and perhaps have more voices to join you in complaining. 

b.  Documentation is important. You don't want this to be a case of "whom do we trust more." If you feel sufficiently strongly about this, get the assistant's answers either recorded (audio-visual) or in (his/her) writing. If the assistant also answers queries by e-mail that's one way to provide documentation. 

c. Be factual, do not exaggerate. When I answer students' questions, sometimes I would deliberately give answers which, from the point of view of an expert, is incomplete. These can be pedagogical decisions made to guide a student's focus in his or her studies. While I am not claiming that your tutor is doing precisely that, it is important you present his/her answers exactly as they are, and not "interpreted by you."

d. Another option is to make this about the system, and not the individual. You say that the tutor has open office hours during which he/she answers questions from all subjects. Maybe that is the problem! Propose to your professor(s) that it may be more efficient to have tutors divided up by specialty and answer questions in those fields only. (At my current institution we have precisely a system set-up so that tutors will not be answering questions out of their expertise.) 
When to tell the professor

As early as you feel comfortable. If I am in charge of tutors, this would be something I want to find out sooner rather than later.