Helly-Bray's theorem for bounded discontinuous functions If a sequence $(\mu_n)_{n\ge 1}$ of probability measures converges weakly to a p.m. $\mu$, then $$ \lim_{n\to \infty} \int f d\mu_n=\int f d\mu $$
for every $f:\mathbb{R}\rightarrow \mathbb{R}$ bounded and with finite set of discontinuity points $D$ such that $\mu(D)=0$.
Can anyone give me a hint? I know a proof in the case where $f$ is continuous using the fact that for every $\varepsilon>0$ there exists $a<b$ such that $\mu((a,b])>1-\varepsilon$ and writing
$$ \left \vert \int f d\mu_n-\int f d\mu \right \vert \le \left \vert \int_{(-\infty,a]} f d\mu_n-\int_{(-\infty,a]} f d\mu \right \vert + \left \vert \int_{(a,b]} f d\mu_n-\int_{(a,b]} f d\mu \right \vert + \left \vert \int_{(b,+\infty)} f d\mu_n-\int_{(b,+\infty)} f d\mu \right \vert $$
Is it a good path?

Provided that you know that the set  discontinuities $D_f$ of $f$ is measurable,  then one may use the Portmanteau theorem as follows:
(i) Claim ($\mu_n\circ f^{-1}\stackrel{n}{\Longrightarrow}\mu\circ f^{-1}$: For any closed set $F\subset \mathbb{R}$, we have
$$f^{-1}(F)\subset\overline{f^{-1}(F)}\subset D_f\cup f^{-1}(F)$$
If $\mu(D_f)=0$ then
$\mu(f^{-1}(F))=\mu(\overline{f^{-1}(F)})$. By the Portmanteau theorem
\begin{align}
\limsup_n\mu_n
(f^{-1}(F))\leq\limsup_n\mu_n(\overline{f^{-1}(F)})\leq
\mu(\overline{f^{-1}(F)})= \mu(f^{-1}(F))
\end{align}
This shows that $\mu_n\circ f^{-1}\stackrel{n}{\Longrightarrow}\mu\circ f^{-1}$.
Now, let  $\phi(x)=((-M)\vee x)\wedge M$ where $M=\|f\|_u$.  As  $f=\phi\circ f$ and $\phi\in\mathcal{C}_b(\mathbb{R})$, by part (i)
\begin{align*}
\int  f\,d\mu_n&=\int \phi\circ f\,d\mu_n=\int \phi \,d\mu_n\circ f^{-1} \xrightarrow{n\rightarrow\infty}\int \phi\,d\mu\circ f^{-1}=\int \phi\circ f\,d\mu=\int f\, d\mu.
\end{align*}
