Is there another adult in your family who you trust who speaks the foreign language? Maybe if your husband isn't present you can ask the other adult to 'chaperone' any conversations between the child and other family members. Or, as another person said, put a rule in place that your child can only converse in that language when your husband is present.
I think it would benefit your child if you find a way to encourage them to learn the language of their heritage. When my grandparents come to Australia they learned English and never spoke their native language around English-speaking people, including us grandchildren. As a result I never learned their language and grew up monolingual. I'm still sad about that.
It's easier for children to pick up a second language when they're young so shielding them from exposure to the language may reduce their ability to learn it in the long run.