$u$ is a harmonic function, and we have $$\Phi(x):=\begin{cases}-\frac{1}{2\pi}\log|x|&&(n=2)\\\\ \frac{1}{n(n-2)\alpha(n)}\frac{1}{|x|^{n-2}}&&(n\geq 3)\end{cases},$$ $$u(x)=\int_{\mathbb{R}^n}\Phi(x-y)f(y)\mathrm{d}y.$$
It is claimed, at the top of page 34, that $$ \left|\int_{\partial B(x,\epsilon)} \Phi(y-x) \frac{\partial u}{\partial\nu}(y) \mathrm{d}S(y) \right| \leq C\epsilon^{n-1}\max\limits_{\partial B(0,\epsilon)}|\Phi|,$$
where $\nu$ is the outward normal from $\partial B(x,\epsilon)$ and $\alpha(n)$ is the volume of the $n$-dimensional unit ball.
How did Evans get this bound?