There is a picture of the stairs up to the upper deck in this thread.
https://www.flyertalk.com/forum/brit...-ba-747-a.html
Will your companion be able to manage the stairs ?
There is also a comment in that thread that BA won't allow someone who needs airport assistance to sit in the upper deck. I appreciate this thread is from 2013 so it may not be the current policy but wanted to flag it up.