Especially for adults, if they’ve made a decision, they have probably struggled long and hard making that decision. As a friend you might question why and how they’ve reached this point or express concern. It is certainly normal to do so. I would just ask that you realize that they’re probably looking for support more than a judgment call from you.
When you do want to weigh in, and inevitably you will, first be educated on the topic. If it’s about hormones go find out something about them before you try and have a discussion. If it’s about your female to male son getting top surgery try to learn about that.
Finally, the most important point of all this do not be condescending and be prepared to listen and respect their viewpoint. This is not to say you have to agree, but you need to show them that you are treating them with respect.
Remember that this affects their life enormously and in ways that are nearly impossible for you to completely understand. You may well empathize with their pain and struggle, but it is hard to get the outlook.