Make sure they didn't just tp in and their clothing just hasn't rezzed yet. Unfortunately, there's not much that can be done about this issue. A number of the add-ons like that are supposed to autohide in a G or M sim. The problem is that if a sim has scripts turned off, that won't work. If they don't notice this, it obviously can cause a problem.
If you choose to say something to somebody with an issue like this, do it in IM. There's less chance of them feeling they have to save face that way. Remain polite (even/especially if they don't). In most cases I've had to deal with in staffing a G rated sim, a polite request is all it takes.
If they indicate they are a 'speshul snoflake' for whom the rules don't apply, then contact a sim/region manager if possible to deal with the issue.
Fill out an AR if you wish. This is especially effective if you get a screen shot with the offending bits clearly showing. I haven't had to do that as most people who don't take care of it right away when asked, get the idea we're serious when they get sent home.