A new player who only has Mordred won't exactly have any need for healing, you're just going to get one-shot...
Bringing Mordred is fine as long you don't use Outrage. That is, if you AAB, please no.
cassiopeia still has healing and defense up.... and they said they have that soul as well
the longer you live, the more PP you gain after all, and a newer player is gonna need PP as well since they likely won't get as many raid tickets as a more experienced player...