Due to anon's request i will have to keep their name a secret.
So for ur answer dear anon.
I myself was actually confused about this subject. YES rigr did betray guren when he was fighting the First but it was on a wimp for rigr from my perspective.
Rigr has always wanted for sikama to notice him , to acknowledge him . Rigr was mad bc sikama used him and abandoned him. So when sikama mocks abt Rigr being weak in early chapters.....Rigr was mad so to prove to sikama that he was strong he tried to eat demon guren mahiru and noya to impress sika...
On the other hand guren and company knew abt Rigr's sikama complex ...he knew Rigr might betray him anytime if need arises so he was prepared with a trump card of his own who was no other than Ferid who had a similar complex towards Rigr. It's a basically use each other kind of partnership between guren and saito and guren and ferid..
Seeing this scene i thought first Guren was surprised to see Rigr who asked for his help e after betraying him but the later scene made me thinking that guren might be actually working with saito even when saito tried to betray him
Yes guren and mahiru were shocked to see saito there but if you notice the nxt sentence the moment saito mentioned abt putting there plan into motion..guren immediately knew what he was referring to as guren brought a chain out of his palm as soon as guren heard abt the plan. Guren always knew saito would appear sooner or later to ask him for help and begin the nxt part of the plan but just didn't know the way he would contact guren which is using kimizuki as a medium for communication thus guren and mahiru's shocked expressions
I hope you will get ur answers from this dear anon😊