Hi lullofish,
I can't give you a complete answer, but here's some information. There is a marked walking route from Lisbon to Fatima, which is a total distance of (very roughly) 150 km. The Camino and the Fatima route are the same until about Santarem, which is a 3 or 4 day walk from Lisbon. I can't tell you anything about the quality of the marking from Santarem to Fatima, but I would say that most of the marking from Lisbon to Santarem is yellow arrows, with only a few Fatima markings.
So that seems like it would be only about 5 or 6 days walking from Lisbon. South of Lisbon, I don't believe there is much marking, so I don't think your friend could take a longer walk by starting further south.
There would also be a possibility of picking a point north of Fatima and walking south. The Camino and the Fatima route join again somewhere north of Coimbra, but that would mean walking on the camino "backwards" and I am not sure about the quality of the Fatima markings from Coimbra to Fatima (or wherever the road south to Fatima diverges from the Camino) .
Hope this makes some sense. Laurie