Thinking about issues of ethics regarding translators (ethics test coming soon!), I have come across a couple of blog entries by Rose Newell (German/English translator):
Read them carefully and note all the different issues treated here: plagiarism, unethical translators claiming to work for other translators found on professional websites, negative proofreaders, etc.
Check her ten points towards ethical proofreading.
Send me/your teacher a summary of what you've learnt from reading the blog posts linked above an refer to the code of ethics relevant to the issues mentioned.