Firstly you need to remember that he is your friend and him being trans shouldn't affect that except if he wants to talk about it.
Don't assume there is one set way to transition and that all people who are trans will want to have hormones, top and bottom surgery (trans people have different levels of dysphoria about their bodies) and that this will happen quickly. Waiting lists on the NHS can be super long.
Terms to not use and better alternatives (the first two you have used):
'Preferred name' - this is just his name.
'Feminine name' - I think generally people who are trans* use 'birth name' or 'assigned name'.
'Born a girl'/'Used to be a girl' - use 'assigned/designated female at birth' or 'raised as a girl'. A lot of trans people say they were never their assigned gender.
Tips for general ally-ness:
If unsure ask about pronouns (and stick to the ones you are asked to use).
Don't talk about 'passing'. Due to their bodies or their finances some people who are trans will not get the body they desire and others don't want to or don't feel they need to fit into cis-centric standards of beauty.
Don't out anyone. EVER. People do not need to know he was assigned female at birth and he gets to chose if he tells someone (and if so, when). Equally if someone comes out to you as trans but hasn't socially transitioned yet then you shouldn't tell anyone. At best you will be breaking their trust, at worst you could be putting them at risk of violence.
Disclaimer: I am not binary trans (I identify as genderqueer) so I may have missed things out and not all people who are trans will feel the same way about everything. But I have friends who are trans and so I'm basing a lot of this on their experiences.