How to support students whom other faculty abuse?
Depending on how your department works there might be something you can do. People don't like their foibles and sins brought to light. Bad things happen in the dark. 

See the caveat, below, but if you have regular faculty meetings, you could raise the issue without naming anyone. "Students are complaining to me of racist/sexist behavior by some faculty member. I think we need to discuss it and whether we need policies to prevent it going forward. It is affecting their learning. It is affecting our reputation." This isn't a formal complaint about a person but it could bring about some discussion and possibly a change in behavior, if the person thinks they get no support from colleagues.

I can't guarantee that, of course. Some jerks will just be jerks. 

But the caveat is that if you are a new faculty member (within, a couple of years of hire) this could be extremely dangerous for you to mention. Some places have a culture and they don't want newcomers to rock the boat in any way. All you will get, in some places, is "shut up shut up shut up". You will need to analyze how faculty generally treat one another before stepping forward. If they are willing to accept bad treatment of students, perhaps they are willing to do so generally. 

In the worst case, start looking for an exit. There are some things over which you have no power and no control as a newcomer. You can find a place where faculty respect one another and the students as well. 



A second option, if you don't feel safe in coming forward in public, is to feed ideas to students to make the issue a public one. Nailing theses to the cathedral door has a clear historical precedent. But this would need to come from students and they may not have thought of the possibility. Having a list of grievances posted on office doors is pretty dramatic. 

But this would depend on some solidarity among students themselves. Some actions can be anonymous. Some can be through a spokesperson. And they can get a conversation going amongst faculty that is necessary.