Listen to them and ask what you can do for them
Don’t hold onto worries about their transition, be open with them about your concerns, and discuss them, rather than bottling things up.
Ask them if they want to go clothes shopping etc with you. Help them with finding what looks right in terms of clothes, hair, make-up.
Most importantly, they are the same person, if you still love them, tell them.
We sometimes revert into out shells, and feel isolated, wrecking ourselves with internalised transphobia (aimed at ourselves), when there is no need, so try and be supportive, and pull them out, and tell them it is ok to be them.