It depends on the person. Think about what makes your friend happy. You could (maybe you already do!) listen to them complain or do something with them to take their mind off things. I think it's cool if you provide support now and then; please remember to take care of yourself as well! If your friend wants to talk, then just hear them out. One of my failings is that I tend to focus more on telling people what to do, rather than providing emotional support. Somebody recently improved my mood (and she might be unaware of that) by sharing an interesting link related to psychology. So I had something amusing to focus on and that helped.
Definitely ask your friend what helps them. I know I personally like to hear my name and pronouns used, because it helps me believe that even if my body is wrong, at least others see it the right way. That's a simple thing that can help me out when my dysphoria is bad. Other than that, distractions tend to be the best solution for me, but some people like to talk about it. Just ask your friend what they prefer.
