Hi Chacharm
I think I'm interpreting your query correctly - you're visiting friends in Buzy near Pau before you start your Camino, and as they have some holiday time they have asked you to stay for a week instead of the weekend you had planned, which would mean you would be starting your Camino 5 days later than intended? And you then need to be in Vienna by 1st June? You would probably still have time to walk from St Jean in that time, but if you want a less pressured pace you could save 3-5 days by starting in Pamplona, depending on how fast you walk. Or start in St Jean and walk as far as you can, or skip a bit later on if you find you're running short of time and want to end in Santiago. Getting to Pamplona from Buzy could be your main challenge, unless your friends are very nice and decide to drive you! By train you would have to go from Pau to Bayonne and then down through Irun - a few changes of train or bus to get to Pamplona. Maybe just go to St Jean and start from there as planned?
Have a wonderful time anyway - Buen Camino!