My reading is that if Lena enters a relationship (something that only happens after potentially sucking on Jeremy's cock) then Louise won't ask her for the threesome because she would be effectively asking her friend to cheat on her now-boyfriend (Ian), something Louise is generally allergic to.
Yes you are correct. So let assume that Louise already said 'no' to Jeremy in regard to this threesome to include Lena, because she also believe that Lena would say 'no' as well. But unfortunately Jeremy was very persuasive so Louise asked Lena anyway only to prove the point that Lena would say 'no', which was correct that Lena eventually said 'no'.
However, this Louise asking Lena scenario, on behalf of Jeremy, for the threesome, was only a setup scenario. So that when the threesome (Louise, Jeremy and one other girl, probably Alison) takes place at Louise's bedroom, and Lena choose to take a peek at the action. If Lena has fetish of threesome or big cock, then she feels regret for turning down the offer.
Having said that, this whole scenario is just a wishful thinking, unless someone tell Evakiss to include this scenario in her game.