It doesn't matter. Waking Rem up was a major goal of them. Waking up Gluttony victims also was one. Rem waking up is huge in that respect. They should also react on Ram and Subaru's future reactions on it.
They should also consider that functionally, a new member will join their camp.
Saving Rem is 50% of the goal of their mission, they should go over it more.
Lastly, they should speculate that means gluttony is reversible and that kiling Rai could have saved Rem, so they should kill Roi probably. (they kept him alive because of that)
All of the above remains true even without memories of her.