My friend decided to prune a Silver Birch, out of the branches where he cut is now a steady flow of what appears to be water. Does anyone know why or what this is. He says the ground is quite wet, but not especially so. Is there anything he needs to do to stop the water flowing out the end of cut branch?

At this time of year the sap is rising, so that is what your friend is seeing.
Probably best to leave it as is, any treatment of the cuts is likely to seal in disease.

Julia wrote in
:


My friend decided to prune a Silver Birch, out of the
branches where he cut is now a steady flow of what appears
to be water. Does anyone know why or what this is. He says
the ground is quite wet, but not especially so. Is there
anything he needs to do to stop the water flowing out the
end of cut branch?

it's sap. the tree is 'bleeding'. it will stop when the tree
is able to grow scar tissue over the pruning wounds. how long
that takes depends on the tree, the time of year, etc.
lee
--
war is peace
freedom is slavery
ignorance is strength
1984-George Orwell

Your friend just mistimed the pruning, birch and maple would be better
off pruned late season (August) because as you have witnessed they
bleed.
Do not apply anything to the wound.