Firstly, would this colleague be as supportive to you if you were in this situation?
If so and you think its the right thing to do then go.
Just say nothing, you're not a mediator and its not up to you to tell either party what they're doing wrong, or take sides.
she only wants your support so just give your support.
Also, let your colleague explain your presence, make sure the supervisor knows your there at her request and that your are totally neutral.
you don't want to get dragged into the politics between the 2 of them.
witness what is said and let them sort out the problem themselves with minimal input.
good luck!!