Welcome to our daily nightmare. First thing I want to say: we have a pretty good fix for this in testingn that will be released soon that should greatly motivate downloading the refund file and reduce these situations to a minimum. We'll hit the user with this in chain -> lightning swaps:

That being said, there is an emergency mechanism if all refund info was lost: here the user needs to extract the preimage directly from the invoice that was used in the swap and with this preimage, the user can craft the refund transaction. We usually help users to do that and have pretty much all custodial lightning services like WoS on speed dial to help the user get their preimage. But again this is quite some manual work, involves multiple rounds of communication and definitely sth we'd ideally not have to do anymore in 2024.

So far we are not aware of any loss of funds because of lost refund info.