If the papacy can be split between the material and spiritual offices (and that's debatable), then one can make the argument that Pope Francis holds the material office only, with his spiritual office being doubtful, due to his personal heresy. If Pope Francis still holds the material office (and he AT LEAST holds the material office, since he has yet to be removed from it), then the papacy is not vacant. If the papacy is not vacant, then one should pray for the pope at mass, seeing that his lack of spiritual office is due to himself alone, through his denial of the faith, and therefore, through prayers, he still has the opportunity to convert and regain his spiritual office. Therefore, prayers at mass are still beneficial to him (in theory).
If one is a pure sedevacantist, they won't agree with the above, as they think there is no pope. Yet since their theory can't be proven with certainty, it is not wrong for them (or anyone else) to pray for the pope either, since an "una cuм" mass can only be a sin if the Church has said so, and/or we know with certainty that pope x is a heretic, which obviously has not been decided ecclesiastically.
Either way, an "una cuм" mass is not wrong. It is never wrong to pray for someone at Mass, unless an authority has forbidden it (FYI...Fr Cekada is not an authority).