So I'm a Spider, So What? - Kumo Desu ga, Nani ka?

[Buffalobiian]

But yeah, perhaps we will learn the details when it's shown why Kumoko (as the pale lady) is working side by side with the demon lord.

I'm going to guess that Kumoko got herself blasted into Vestiges or something from that attack, so now is some form of empty slave to Ariel. Ariel is destroying the world due to Body's influence on her. At some point, Kumoko is going to come back in full force.

So pretty much I don't think white girl is Kumoko going "Ariel's cool, I'll tag along for a bit".