Sounds like you have a small company without a formal HR Department and little experience on the subject. So do a quick read on a Google search on the topic to get some ‘Do’s & Don’ts’ on how to conduct yourself so not to increase your liable. Get them aside and politely discuss the the issue. If needed, get the Foreman to sit in on the discussion. Afterall, the laborer works for the employer and the employer works for the client and one employe/laborer can devastate a project and the company’s reputation.
Good luck!