Louise reaches out to Ian simply because she's bored and he turned out to be a likable guy. That can very well be all the reason she has -- to chat with someone who was friendly with her to kill boredom, and their interaction can very well end at just that.If Lena is Ians girlfriend, very happy with him and not on the slut path displaying hypocritical/sexual aggressive behaviour to Louise, and also on Louises good terms (best done by bullying Stan), then Louise shouldn't have a reason to contact Ian and sent him a selfie.
It's only when Ian instigates some heavy-handed flirting that she really goes along with it.