\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\begin{document}
{
\fontfamily{phv}\fontseries{mc}\selectfont
THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%
}
\DeclareFontSeriesChangeRule {mc}{b}{bc}{}
\DeclareFontSeriesChangeRule {m} {c}{c}{mc}
\DeclareFontSeriesChangeRule {m}{mc}{c}{mc}
{\fontfamily{phv}\fontseries{c}\selectfont %grr
THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%
}
{\fontfamily{phv}\fontseriesforce{mc}% this works
\selectfont
THIS IS SAMPLE TEXT \textbf{THIS IS SAMPLE TEXT}%