There is a jewish tailor in my neighborhood who took a shatnez course, but is not certified, so he does not advertise about shatnez checking. There are non-frum Jews who come in to get their suits tailored and he knows that these suits have shatnez almost all the time. What he told me that he does is that he removes the shatnez from the suit and repairs it without the customer knowing, with them thinking that he just tailored the suit. Is this halachically permissible?
As long as it causes no damage at all to the suit and doesn’t decrease it’s value in any way, this would be permitted. However it would be preferable to inform them that he is providing this service.
In addition, if he does not remove the shatnez it would be problematic to tailor their clothes as this would be aiding them in wearing forbidden clothing, מסייע בידי עוברי עבירה.