PG student in my department with whom I have a good relationship has come to me and told me that she was asked to do some editorial/RA work by a colleague. The work was unpaid, and when she did not do it perfectly he was cross and criticised her. I think she was more upset about the criticism than she was (initially) about being asked to do the unpaid task in the first place, which I think would have taken a few hours or less.
Now I have heard that this same colleague has asked other PG students - all women - to do similar unpaid editorial/RA work. Some did the work, fearing the repercussions when he wrote them references. One refused to do the work and then got a lukewarm reference.
When I heard all of this I wanted to immediately report it to my head of department, but I didn't as the PGs didn't want to go down that route. I feel my hands are tied - the PGs (understandably) are very leery of 'reporting' him because they fear that there will be repercussions for them, they will look like troublemakers etc. As well as the blatant abuse of power by my colleague, I am dismayed because I remember so well the feeling of having to stay on the right side of academics as a PG because you never knew when they would have power of life or death job or no-job over you.
Colleague is senior to me. When I first arrived I had to fend off a few requests from him to do work that should have been his job but that he was trying to get me to do. I have spoken up a few times fairly robustly in meetings against decisions he has made - courteously and not in an attacking way, just to show I am not a doormat - and thankfully no more requests have come my way. I don't like him, but I have to work with him, and there is a fair amount of overlap in our areas so I don't want to antagonise him unnecessarily. I'd just like him to stop asking grown women to do unpaid wifework for him 
Has anyone had a similar experience in their department? Any HoDs have advice on how best to resolve this? I will feel like a rubbish human being unless I try and do something. I just don't know what I can do.