Professor does not care about cheating, what should TA do?
I realize that the following advice may be very unsatisfying. But you may need to hear it, just to save yourself from grief. I hope it helps. 

It may be that you have done all that you can do without harming your own future. You need to judge that, of course, but getting between a new professor and the administration can be uncomfortable at best and career ending at worst. 

But you can, of course, vow to be a better actor than this professor when it comes your turn to be in the driver's seat. 



There are a lot of possible "reasons" for the situation you are in, none of them especially valid, but still possibly determinant. 

Perhaps the department doesn't care a lot about these students or this course, as it doesn't sound like they are in the major. 

Perhaps the professor is so tied up in research that he just doesn't care about (or have time for) teaching. This is true, in fact, for some untenured professors. 

Perhaps the professor is seen as a future superstar who is immune from all criticism. 



If you weren't alone, and had other TAs with similar concerns then you would be in a better position to bring the situation to the attention of higher-ups. If you are friendly with another professor who sees the problem and also objects to it, you could work through him/her. 

If your duties included more than grading, such as leading small breakout sections, you could, perhaps, work with your smaller group to get some local change. But, if it is the professor's job to teach them and he refuses to do it effectively then there isn't a lot you can do. 

But as a sole TA with only grading responsibilities you don't have a lot of options that won't come back to your own disadvantage. 



Certainly you can bring it to the attention of the undergrad coordinator, with evidence. Whether to do it without the knowledge of the prof or not is a (risky) judgement call. But if a solution is to be found, you probably won't be part of creating it. It would be a faculty committee that would have authority and be able to effect change. But you would likely be in the center of any controversy that was generated. As a doctoral student, it feels pretty unsafe to me. 

Look to the future, and learn from the past. Do your best but don't put yourself at risk unnecessarily.