"Can the one who wants to move back in prevent a sale as they are arguing that it may be protected by the FHPA and any sale would render them effectively homeless and unable to purchase another property."
Nothing to do with anything. The FHPA is not a cure-all or cure anything for this situation. They may as well call on the Man in the Moon. The biggest issue is getting them to engage - if they won't engage, won't sign anything, won't agree to the sale etc.,etc the other owner can't sell without them. That is how they prevent the sale.
mf