It could be morally good to return some grants if there is a good theory on how this will lead to better results for the people involved and their families, for the communities they are part of (primarily, the EA community), for society, and for the civilisation.
Some deontological motives and considerations could be a part of such a theory. For example, as other people already mentioned in this discussion, returning grants could send a valuable signal to the EA community and to society.
However, it seems to me, the framing of the question "Under what conditions should FTX grantees voluntarily return their grants?" hints at the possibility of some hard-and-fast deontological algorithm for deciding when grants should be returned. I don't think such an algorithm exists. The theories for why returning funds would be good should be far more nuanced, and applicable to very narrow strata of grantees and victims respectively (perhaps even down to individual grantees and individual victims), rather than large strata such as "all grantees" and "all victims", or even "1% of victims who were affected the most in terms of the portion of their net worth that was destroyed".
Considering the above, I think just returning money to FTXFF (or another pool of money) would be ineffective. And even creating a short-lived organisation to administer claims for returns from the victims will be ineffective, too (especially considering the opportunity cost for people who can quickly create locally-effective organisations of this sort: I believe such people have much more valuable things to organise, from the EA perspective).
I think a solution that could be low-investment and also relatively effective is organising a forum where individual victims share their stories and ask for help, and individual grantees can come and respond, assessing their own situation and the situation of the victim, that is, building "a good theory". And then publicising this forum among both the victims and the grantees. This also doesn't mean grantees should return their entire grants, they may help a little, according to their situation and the situation of the particular victim. However, one complication with this solution might be: how could the stories of the victims be verified?
In this setup, grantees should also consider the implications of their decisions for the community and society, not just themselves and the victim. While the latter are highly individual, the former are mostly shared. So it would make sense for some people who are experts in community strategy, sociology, and ethics to write a few essays on this topic that grantees would be advised to read before visiting the forum. (I'm not such an expert.) Of course, individual grantees would still be free to form their own sub-theory regarding these "high-level implications", according to their own understanding of the community strategy and ethics.
I think that Fund recipients should return the funds only if legally required. The utility derived from money in the hands of recipients (assuming the Fund is competent in its deployments) far exceeds that of the average FTX victim. Fund recipients are not morally responsible for the wrongdoing of FTX, and should continue to use resources in the high EV way that ostensibly provided the bases for the grants.
The couple arguments against this do not likely hold up against the vast utility discrepancies from resource allocations...
One would be that EA has better press because it intentionally deprives fund recipients who are not legally obligated to lose funding. I don't think it reflects well on EA if we encourage defunding programs we deem extremely high value to pay debts without legal basis.
Another would be that by not disgorging the entirety of a fraudulent actor's benefits (by it being sheltered by grants) we potentially incentivize future fraud by EAs. I think given the endless recriminations and loss, it is hard to think that a silver lining of some ill-gotten gains being put to good use would encourage future bad behaviors.
The dedication of resources to the Fund recipients is likely very high EV and I think we should encourage these recipients to retain these resources for their purpose if legally permitted.
If that is the state of the law and they have a legal obligation to return it, they should. I just would not endorse returning if not legally obligated to.