Thanks very much for all your replies, initially we had planned on starting our walking a little further back in Samos taking 7 days to reach Santiago (average of 18km a day) , now based on your advice and wanting to go to Finisterre, we are going to start our walking in Sarria, arriving in Santiago on the 7th for pilgrims mass, then on the 8th we will travel by bus to Finisterre, spend the evening of the 8th (hopefully see a sunset at the light house) and some of the 9th there, coming back to Santiago by bus on the 9th in plenty time for our friend to make her midday flight on the 10th and the rest of us will have plenty time to explore Santiago.
Do you suggest that we book accommodation for when we are walking? I was under the impression that you couldn't book the municpal Alberuges which we had planned on staying in?