First make sure that you are up on the halachot, e.g. maybe there is a tsad heter. Perhaps the person relies on a minority opinion (if there is one, IDK).
Figure out the person’s religious level. You already said that s/he probably carried on Shabbat. Is there a reliable eruv? There level is important to know if they really care, so that you know if they are approachable.
Find someone whom they will listen. If not, proceed with the utmost of care. Show them genuine concern for them.
Maybe you should ask the Rav of your shul.