I agree with Mental Magpie. We keep a collection of old but clean and decent suits for guests. Perhaps they aren't nice enough to provoke "theft" but there will likely be one in the collection that fits any given guest, and it goes in the washer when the guest is done. Not a problem for me as host or guest.
Why aren't you getting the suit back from her, spun and in its little plastic bag, before you leave the Y?
You could make your spare one of your old, ratty suits you can almost see through, one that's about to dissolve, demand it back when you leave the Y, and never have any other suit available. Or stop carrying an extra suit. Or just stop going with her.