Bilbao to O Cebreiro or Sarria

Bus? You can take a bus practically anywhere in Spain for very little money.
The bus station in Bilbao is easy to get around if you speak a little Spanish, imo.

There is a bus from Bilbao to Sarria for around 44 Euro on ALSA website.
They don't appear to run to O'Cebreiro, but someone surely does, as it is a tourist site.

There are many small bus companies in Spain, and different ones take different routes.
I'd be really surprised if she couldn't get to O'Cebreiro.

The buses are very nice, too. BIG old windows, great for site seeing, clean and spacious.
Good luck!

You can take a train with one connection. See http://www.renfe.com/EN/viajeros/index.html

Search Bilbao to Lugo. When the search comes up without any direct trains, click the link for journey with a change of trains. Most of the trains go through Sarria to get to Lugo. Click the train name in the second to last column to see the itinerary.

No bus actually goes to O Cebreiro - it stops down the hill at Pedrafita about 5km from Cebreiro.
You can get an Alsa bus from Bilbao to PEDRAFITA.