Caminho Portugues by wheelchair

Bless you for thinking of your friend, but I just don't think the terrain is up for wheelchair travel. I only walked one day along the coast, and as I recall, large parts of it were like boardwalk. Inland, it was a lot of cobblestones and hard packed dirt, with only a few, but very steep inclines. I recall a poor Polish woman with bad knees just in tremendous pain trying to inch her way down a hill, and can't imagine being able to control a wheelchair on it.
Rattling over boardwalk would definitely be easier than cobblestones, but I only saw one day of the coastal. If you'd like me to create a group of surfaces I hiked on for you to see and judge for yourself, I'd be happy to do so.

Hi Welmoed, I walked from Porto in 2014 and met a husband and wife from Germany, both on wheelchairs and doing the Camino, aided by their young cousin, started from Porto too. I met them in Ramallosa, a small town just passed Baiona. We stayed at the Apostle Sisters convent (10 euro private room! Beautiful facility and court yards). Ramallosa was where they stopped as the Camino further on is not very suitable for wheel chairs?
They also told me they plan to set up a website to share their experience and to help others, I hope you could find it (probably in German). I think they also have somebody with a van to transport them in some sections. We went for a stroll in the beach with boardwalks and he managed well. Sorry I couldn't locate their contact info.
Bom Caminho!

I agree the coastal route would be quite a challenge. I recall some steep trails with ruts in the rocks (from Roman chariots I presume.)

Having cycled the Portugues - inland route - and having had to use a wheelchair myself in the past - i would say - research the route b4 you go - viamichelin maps have the camino routes on the 'simple' type map (drop-down on right side of map page - shaped like a map) so that might be a good place to start. I haven't done the coastal up from Porto - but on the inland route there was a whole section from Ponte de Lima that you just would NOT want to do (worst day of my life....) Any really rough offroad section you should be able to find a small road instead. The bike wheelchair - or even make your own 'tougher' version may be a better bet - one cycling the other helping push up the hills or restrain it on downhills, also it means you can swap for a rest...! We had a trailer (one wheel) behind the bike and it was fine. But i would recommend Schwalbe marathon plus tyres on any bike and trailer you use - they are bombproof. Some of the trails are steep and some rocky - but i would think if you and your friend are up for a challenging journey - then go for it! Make sure you have some contingency - in funds if you need to stop for a hotel due to heavy rain or a really tough day, and know where possible train/bus connections are if you cannot continue, and always plenty of water and dried fruit/nuts in case it takes you a lot longer on a tough section than you expect.
But i expect you will have a great adventure - bom camino and good luck!