This introduces a key mirror, whatever you pass to this key will get mirrored. In your figure this amounts to saying
\tikzset{mirror={\draw (1,1) -- (2.5,3) -- (1,4);}}
There are styles mirrored and unmirrored, which can be used to change the transformation laws, introducing offsets or also name prefixes, say.
\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[mirror/.code={\tikzset{mirrored stuff/.code={#1}}
\begin{scope}[unmirrored]
\tikzset{mirrored stuff}
\end{scope}
\begin{scope}[mirrored]
\tikzset{mirrored stuff}
\end{scope}},mirrored/.style={xscale=-1},unmirrored/.style={}]
\draw [help lines, dashed] (0,0) grid(5,4);
\tikzset{mirrored/.append style={xshift=-5cm},
mirror={\draw (1,1) -- (2.5,3) -- (1,4);}}
\draw[red] (2.5,1) node[below]{A} -- (2.5,4) node[above]{B};
\end{tikzpicture}
\end{document}

Please note that there are much more versatile options available like this one which I found after completing the example.