If Lena and Ian aren't together, then Lena has option in the epilogue to indicate her willingness to go or lack thereof, and it's stored in a variable.
Python:
if v10_ian_left or v11_lena_breakup:
l "About that..."
menu v11perryinvite:
"I'll go":
$ v11_perry_invite = 2
l "I guess it's alright if I join after all..."
// ...
"I haven't decided yet":
$ v11_perry_invite = 1
l "I don't know, I haven't decided yet."
// ...
"I don't think I'll go":
$ v11_perry_invite = 0
l "Actually, I don't think I'll go."
// ...
I think it'd be just as weird to do all this song and dance if in the end player isn't given any choice whether they attend this or not. Keep on mind that Lena not attending opens her schedule to interactions with her other routes (Mike, Robert, Stan, possibly Jeremy, not sure if he's coming) and that can be a way to start wrapping these up. So between this and Ian being potentially paired with someone else, there isn't necessarily much sense in forcing Lena to go, if she hasn't developed close connection with people who are going to be there.
Lena is at best, wavering if she didn't want to go. And don't forget outing with Emma is completely optional. Lena can very well go shopping alone, and then it'd be quite weird for her to change her mind out of the blue and ignoring player's input.