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!