There has been quite some research into the influence of a second language on a speaker's first language. A collection of papers on these topics has been edited by Cook (Effects of the Second Language on the First; 2003).
Research suggests that even speakers who remain in their L1 community but acquire advanced skills in an L2 are influenced in their pronounciation of their mother tongue by the pronunciation of similar sounds in their second language. (Lord: Second Language Acquisition and First Language Phonological Modification; 2008)
More generally, Pavlenko: L2 Influence on L1 in Late Bilingualism (2000) claims to provide evidence that even in adult L2 learning and use, L2 influences L1 phonology, morphosyntax, lexis, semantics, pragmatics, rhetoric, and conceptual representations.
Any of these effects will be viewed as detrimental by the speaker themselves and/or by their L1 community as soon as it leads to noticeable divergence from the L1 norm.