Walnut tree trouble

I have friend that has a young walnut sapling in his garden but has found the outer leaves are going brown. Does anyone know what could cause this?

 

Might be just autumn approaching? A lot of trees round here have already started leaf shedding and dying.

 

Erm, yes, I guess that could be it!

 

If it's been exceptionally dry, it's probably suffering from drought! I believe that walnuts are quite thirsty trees.....