Laurie isn’t upset about Valentin, she’s upset about Jaclyn
Valentin himself is a non-issue in this case. It is the feeling of betrayal by a friend, who led you to believe that they were rooting for you to get with a guy, and then hooking up with him herself in secret.
I have been a part of and an observer to situations like these. I mean, sure, Jaclyn has every right to hook up with whoever tf she wants to. If Jaclyn told Laurie that she wanted to hook up with Valentin, it would not necessarily offend her. It is the pretense, going behind her back, and making a fool out of her that has hurt her. She has been led on to believe that she should pursue something with the guy, over a period of many days, by Jaclyn. It is the dishonesty of the whole thing that has rightfully upset Laurie, and now all the masks are off.
She is done pretending. I can’t believe how the other two gaslit her when she expressed the truth, because they are so used to false pretence. Acting like Laurie is jealous of the guy’s attention, when it is not the case; she is genuinely hurt by Jaclyn’s treachery.