How can I support my transitioning friend?

[One2Three4Five6]

I'm not unaware of LGBT by any means, but a close friend has recently told me they wish to transition from M to F.
Obviously I want to support my friend in this life changing decision, but how is best to do that.
Anyone on here that has transitioned/is transitioning, what is the one best thing a friend did for you?
Or what do you wish they had done?
Any and all advice is appreciated.

I have already told my friend she has my full support, and have lent her a dress until she gets her own wardrobe up and running.
I have listened and made sure she knows I believe in the need to transition so she can stop pretending to be someone she is not.
She asked me to organise her first girly night out, so she can introduce herself to the world as she is meant to be, I have done this, with friends who have never known her as a M, so she can simply just be F without having to explain herself (we talked about the friends I would organise for her first 'outing' and she wanted girls who didn't know her as M)

Also, any advice on the best way of telling my own children about the change would be appreciated.
I want to be 100% honest with them all, DH wants to be honest with the teens, but not our 7yo.
He thinks we can introduce our friend as a F without our DD realising, at this point I do not believe this is possible. I believe that DD will know it is our 'M' friend dressed as a woman, and will speak as such, which I feel would be worse for our friend.
I believe our friend will understand that our DD will have questions, but surely those questions would be better coming from DD knowing the situation fully?

Again, any advice will be appreciated.

My advice (as a trans person)
The best thing you can do is to validate their gender ("You go girl!" [Name], using the pronouns that they want you to etc.).

Also, any advice on the best way of telling my own children about the change would be appreciated.
It depends on how old your children are. You said you had teenagers. The odds are that they will know what transgender people are. Sit down with them, and ask them if they know what transgender people are. Then, if they don't explain to them what it means. Then tell them something like "You know [their old name]? well, they are a transgender woman and now go by [name] and use [pronouns]. Please call them that." With your younger kid, definitely tell them. I find that actually a lot of kids do get it. You could say something like "sometimes some people are born with a girl's brain in a boy's body (reductive binary terms I know, but they are kids) and they are not happy about that, so they take medicine and change how they look/their name/pronouns to feel better." If you do as your DH suggests, it will probably cause a lot of confusion for your kid and distress for your friend.
I believe our friend will understand that our DD will have questions, but surely those questions would be better coming from DD knowing the situation fully?

@EthanAdvice
I forgot the next bit.
You are right in saying that the questions would be better if your child knew the situation.