welcome to the forum i would think you would need a Lasting Power of Attorney to be able to complete BR for your friend I know RevivaUK , one of the forum experts handle LPAs for people who are no longer in the country and i presume it would be a similar situation, it may be worth giving them a call
Dave
Don't worry or know that worrying is as effective as trying to solve an algebra equation by chewing bubble gum.(Baz Lurhman) RevivaUK and Paul Johns helped me through it all i can't recommend them enough!!
It mentions that some prisons are able to facilitate the prisoner declaring themselves BR and that they should ask at the prison.
I would suggest that your friend speaks someone at the prison to see if it is possible for them to declare themselves BR. If not then you should look into getting the Power of Attorney for your friend and you can then declare BR on their behalf.