Agree with the other posters. The lady is still there. and the history says that all concerned have little difficulty with swopping partners and keeping things fluid. Unlikely to change that.
If your gentle friend is assertive enough, I suggest she puts all the bathroom stuff in a box and all the clothes in one of those compressed bags. Get someone to help her put them in the attic or the garage when her chap is elsewhere
. If he questions the missing items (which he mightn't) just say she needs the space. Its all ready for collection at any time. Stay calm, but vague, as though you were talking about childrens toys.
If the owner of the stuff turns up with a key to the house, change the locks after she leaves. Offer coffee, biscuits, sandwich. Be friendly. If she doesn't have a key, welcome her in with tea or whatever and be obliging. Don't offer alcohol and ensure she make an appointment to return. Friendly friendly friendly and helpful but firm.
Good luck.