It would need to be a piece of wood rough enough that birds can walk on it, not so smooth they slide back down into the water. (Forgot to say that the first time.)
Is there a way for the birds to climb back out if they fall in?
If there is not an easy way out, maybe suggest that your friend put a long piece of wood with one end in the water and one out, at a gradual slant, so birds can stand on it to drink and can climb back out if they fall. If that...