Mrs Bowden is a non-canonical character, created for the 2022 miniseries Why Didn't They Ask Evans?, an adaptation of the Agatha Christie novel of the same name. She runs the dry-cleaner's in town.
Spoiler warning: A spoiler is announced! The following section contains details about the plot of Why Didn't They Ask Evans?.
Mrs Bowden is in charge of cleaning Lady Marcham's dress, which Frankie had borrowed and Bobby vomited on due to poisoning, as well as of Marchams' driver's uniform which suffered from mud splash. This gives Frankie the idea to use the uniform to disguise Bobby as her driver when she takes up residence at Merroway Court and Bobby refuses to go back to London.
Spoilers end here.