A close friend is transitioning, how can I be supportive? [View all]

My wife and I have a long time very very close friend who is transitioning from a man to a transgender female at the age of 53. One of the most amazing people I've ever met, a true genius in too many ways to count, and the most kind and generous person I've ever known. Really proud of them, but I'd be lying if I said I wasn't taken aback by their transitioning. Mostly just because we've known him as a man for 3 decades. I realize my discomfort takes nothing to overcome compared to how many obstacles this person is likely to face.

What things can I tell them to help? I mean, we're 100% fully accepting and supportive, but I want to know what they might want to hear from us specifically? They've recently made public that they are transitioning, so they've obviously given a lot of thought and I just want to make sure I don't inadvertently stick my foot in my mouth. How can I help/not help?