Too many of these solutions look like they'd come home to roost at Lynne's house or impact her own welfare or comfort.

I don't suppose you could send an advertisement to the local newspaper that the house is for lease (name a reasonable low price, to encourage queries), and give the girlfriend's phone number? If there is interest in renting (or buying) perhaps bringing it to his attention might encourage him to rent (hopefully to someone you could stand) or sell.

SRS