My coworker is transitioning plus some stuff.

I'll start by saying I'm new to managing managers. I'm in one of those middle management positions. My question is as follows. I was hired at the same time as "Mark" and all was well, we became friends and even had some social dates including times at my house chillin. Mark and I grew together in the workplace for a fairly short time, like 8 months. Mark is in his 20s and I'm I'm my 40s. There was some shit that happened with our boss at the time. Our boss got fired. Neither of us was involved and there's basically no drama about that except that everyone knows he got fired. I became the boss of Mark and the department we work in. I went through an interview process and Mark was always supportive. Mark is now transitioning to be a woman. Let's call her Mindy. I miss Mark. I support the transition. I'm in a new role as boss and I don't want to be selfish but I'm mourning the loss of Mark. Mindy is so moody and I'm having trouble relating. I miss my friend. Btw I'm female if that matters. How do I clear the decks and relate to Mindy when she feels so different but is the same person? I know it's not about me but I do have feelings about it.