When the Clicker gets stuck in the spa's electric system, the tanning booth turns a man into a woman when he uses it. However, at no point does "she" realize it, nor is the transformation undone.
During the "boobs" switch sequence Linda first is wholly naked but then only half naked as the framing clearly reveals her wearing workout bottom and leggings, without having getting dressed in between those images.
Linda is pleasuring herself, albeit quietly, just outside the shower area; where the husband and wife are having sex, thanks to her inducing both with the Clicker. They should have noticed her presence.
How they covered up everyone in the gym area having an orgy against their better judgment, after they all came to their senses after the Clicker was stopped.