Teaching using bad material This semester, due to an increase of students enrolled, I was assigned to teach an introductory course. The course had been previously taught by two lecturers, who taught a different half of the course. The two do not know well the other half, so they developed the teaching material independently. This semester two lecturers were added and each of us teach the whole course. 

It turns out that the slides for the second part are very superficial and contain some serious mistakes and omissions, and so do the tutorials. Mind that the errors are on basic definitions upon which the rest of the material is developed. Also the textbook is horrible because it is aimed at business students and is very wordy with many examples but little clear explanation and definitions. 

I spoke to the lecturer in charge of the first part, who is also course coordinator, and we agreed that it was too late to change the slides and we would do something for the next year. However, the other lecturer must have been informed of my complaints and turned nasty to me, even though I have been careful not to criticise the person but only the material. Therefore, I really do not feel like helping them with comments and suggestions. Mind that it is not an ego problem here, I don't give a damn about proving my colleagues wrong, I only want to do my job and move on.

In any case, I have now to teach five weeks of material containing mistakes. Should I follow the material and teach wrong things or should I warn the students of the mistakes on the slides and teach the right stuff? 

Personally I would prefer the second option (for ethical and selfdignity reasons) but that may create problems in the exam when students have to answer questions on the wrong defintions. Also, the students may be confused and the reputation of the course would be damaged. Lastly, I do not want to talk to the head of department now because it is too late and we all know what happens to whistleblowers.
As a compromise, given you are staying with the existing slides, consider saying things like "For purposes of this course, the definition of X is [what the slides say] but many people use [more conventional definition]". That way, the students know what they should assume during the exam, but have been warned that they will see different definitions in the future.

In thinking about contributing to updating the slides, you would be doing it to help future students, not to help the instructor who wrote them.