Oh I can absolutely understand why parents have a sense of grief. My mother could not have been more accepting of me, but even she went through a grief process for years. Even if one has no doubt that their child would benefit from transition, no one wants to see them suffer terrible distress, complications, or to become a target for discrimination or mistreatment.
When it comes to the possible side effects of treatments, for me it was a matter of balancing them. I had tried to live a happy life without undergoing anything physical, and hadn't been able to. I was desperate and suicidal, even to the point that the presence of a loving and supportive parent felt like a trap - I couldn't live but I couldn't die, and inflict that trauma on her. I know now what a perverse way of looking at things that was, but when you are so desperate for your unhappiness to end your thinking can become very warped.
So to me, the risk of hormones, surgery etc. were dwarfed by the reality of my existence and the unhappiness I felt, and had felt all of my life, and the need to keep going and live my life to the fullest, for the sake of those who loved me.
I can't speak for every trans person, or person suffering sex dysphoria. It may be that the young person you talk about has the same experiences as me, and has simply grown up in a different era. Had I been born more recently, I am sure I would be involved in online trans spaces and potentially viewed by my family as simply being influenced by my peers. Equally, perhaps the increased exposure to trans people that young people have today, has helped to develop an idea that would not have otherwise existed. It is impossible not to be shaped by the environment in which we are surrounded, and so difficult to determine the degree to which these factors have influenced us.
I fully understand the very real worry that people will make these decisions and live to regret it. How are we to distinguish between those for whom it will ultimately be harmful, and those for whom it can be life saving? That's not an easy question, and something I don't think anyone has gotten a handle on.
If it were my child I would allow myself to grieve, to cry, and to struggle with what they were going through and what could come to pass. I would also tell them that there was nothing they could do or be that would stop me from loving them. I think you can provide unconditional love, without agreeing with the course of action that your child takes.
I would try to establish an environment where the young person could talk to me, without fear of judgement or lecturing, in the hopes they would explore their feelings with me. By that, I don't mean blindly agreeing with everything someone says, but rather addressing their thoughts and feelings with empathy, genuine curiosity, and without judgement.
If I had specific concerns about things I thought indicated this wasn't the right path for them, I would try and talk to them about that. I'd explain what I feared could happen, and why I had these concerns. I would emphasise that my attempts to explore these concerns were motivated only by my desires for what was best for them, and not by a desire to control them or dictate their life choices.
Some young people will still be angry and feel deeply defensive. That's not easy for any parent to navigate, but I don't believe that a genuine, kind and empathetic discussion is likely to lead to the irreparable break down of a relationship. However, I do think that approaching it in a dictatorial, angry or controlling way could alienate a young person, even if that anger came from a place of love and concern.
When engaging with people I love on a matter upon which we deeply disagree, I try to remind myself that each of us has unique experiences and ways of seeing the world. It is easy to see a behaviour, and assume that the underlying motivation is one that would cause us to behave in such a manner ourselves. I don't think that is helpful though, and it leads to assumptions and the breaking down of good communication.
I hope this helps, I'm sorry it might not be as concrete as you were hoping for, but it is hard to speak in specifics without knowing the people involved.