It would really depend on where in the Metroid timeline SSBU(somehow) exists. If it’s pre-Prime 3 he’d probably be on Dark Samus’ side. If it’s post, then he would have had the experience of being controlled and seeing his Homeworld invaded, first by her and phazon, then by the Federation as a direct result of that. Ridley getting controlled by someone? We already know the pirates did not appreciate being corrupted(until they were). Now that he’s free, I imagine he’d be fucking pissed. Dark Samus probably traumatized both him and Samus so thoroughly I truly do think they would hate her more than each other, and damn do I want to see that teamup.
The second the two of them finish her off, both exhausted, staring at her dissipating corpse to make sure she doesn’t come back. Without looking up, Ridley prepares to stab Samus in the back with his tail but gets his brains blasted out by Samus, who also didn’t flinch

