The details add a bit of hassle.
How prepared is your friend to rough it? If so he/she could wait on near side of the main road 400m from the terminal for a bus going to Trang. Could be a bummer if raining or real hot (compared to normal hot).
Easier but a bit dearer (150 - 350 bath maybe) to catch the airport shuttle bus or a taxi to the bus Krabi bus station which frustratingly is about 5km in the opposite direction to Trang.
No doubt one of the airport taxis could take your friend to Trang but that would be pretty expensive. Hell they are 600 odd just to Ao Nang!
I've been told there is a travel agent at Trang bus station who can arrange transport to the pier and then across to the island.
If the place is no longer there, there is a whole bunch of travel agents about 4km away, down opposite the railway station who can do this.
Most of the resort longtails leave before mid-day - if your friend is later than this he/she may still be able to make the public longtail which goes across to the fishing village on the east coast which is maybe a 15 minute walk across to most of the resorts on the west coast. A motor cycle taxi would be pretty cheap.
If it is too late for the public boat, the travel agent may be able to arrange a longtail charter, but this won't be cheap.
Trang is a nice town to spend the night.