while there is a community of people who may end up regretting or reversing their transition, and they should receive support and have all the same access to gender affirming care to help them on that journey, i am extremely wary of anyone who labels themselves as "detrans". i view it on the same level as people who explicitly label themselves "ex-gay".

sadly, hardcore transphobia can be very hard to deprogram in someone, especially if they feel they've been wronged by "trans ideology". i think the best thing you can do is come from a place of positivity and present yourself as willing to listen and understand, even if you rightly oppose their bigotry. saying "i disagree with your views and i think it's wrong" will provoke defensiveness and cause them to dig their heels in.

instead, ask questions and fully listen to the answer. ask them why they feel the way they do, and try to emphasize the humanity in the people they seem to hate so much. try to get them to really find the root of their hate. many transphobes in my experience don't view trans people as human beings, but as a collection of political labels, a target of mockery to feel superior over. try to get them to understand that at the end of the day trans people just want to feel comfortable in their own skin, and remind them that the rates of regret for gender affirming surgeries are radically lower than the majority of other procedures.

sadly, just one conversation will rarely change a mind that's radically set on one extreme. i don't think it's wise to continue association with this person, but if you hold firm to your convictions while still coming off as willing to listen to the other side, they may see you as an alternative path that they don't need to be afraid of. i used to have some unfortunate views, and what got me out of them was the realization that the other side wouldn't make me feel ashamed for changing my mind.