Formatting et al in biblatex

Probably the simplest way is to define:

\DefineBibliographyStrings{english}{%
  andothers = {\textbf{et al}\adddot}
}

MWE:

\documentclass{article}

\usepackage[maxcitenames = 2]{biblatex}

\DefineBibliographyStrings{english}{%
  andothers = {\textbf{et al}\adddot}
}

\addbibresource{biblatex-examples.bib}

\begin{document}

\citeauthor{companion}

\printbibliography

\end{document} 

Output:

It should be enough the redefine the name:andothers macro.

\renewbibmacro*{name:andothers}{%
  \ifboolexpr{
    test {\ifnumequal{\value{listcount}}{\value{liststop}}}
    and
    test \ifmorenames
  }
    {\ifnumgreater{\value{liststop}}{1}
       {\finalandcomma}
       {}%
     \andothersdelim\bibstring[\mkandothers]{andothers}}
    {}}

The only change to the default is that we have \bibstring[\mkandothers]{andothers}, so that the 'andothers' bibstring is wrapped in \mkandothers, that can be defined as

\newcommand*{\mkandothers}{\mkbibemph}

In total

\documentclass{article}

\usepackage[maxcitenames = 2]{biblatex}

\addbibresource{biblatex-examples.bib}  

\newcommand*{\mkandothers}{\mkbibemph}
\renewbibmacro*{name:andothers}{%
  \ifboolexpr{
    test {\ifnumequal{\value{listcount}}{\value{liststop}}}
    and
    test \ifmorenames
  }
    {\ifnumgreater{\value{liststop}}{1}
       {\finalandcomma}
       {}%
     \andothersdelim\bibstring[\mkandothers]{andothers}}
    {}}

\begin{document}
\citeauthor{companion}

\printbibliography
\end{document}