I have 4 DDs. I'd probably let my 17 year old DD1 go, and I'd let me 15 year old DD2 go to EITHER a party with alcohol where I collected her, OR a sleepover without alcohol. The combination of the two is a disaster waiting to happen.
Do you know the parents/kids going well? If so, could you allow her to go but pick her up at, say 11? As a compromise? That way she's not sleeping over, and she will be curtailed in drinking as 'mum will see'? I think that would be how I'd play it.
Credit to your DD for being honest and telling you that there will be alcohol and boys. She sounds very sensible - my eldest went through a lying stage in around year 9 and ultimately she got trusted far less as I couldn't be sure a 'sleepover at friends' was what she said it was. That trust took a long time to rebuild.
I also always advocate a safe exit plan for teens to allow escape without fear of embarrassment. Tell her if she calls and says, for example 'I left my hair straighteners on' or some other innocent sounding pre agreed phrase you will instantly ask her if she wants collecting urgently, no consequences.
My eldest has used this before when things got out of hand at a party and the group had gone into the town to a bar, and drugs had been talked about. I was very angry that she'd lied about her plans and had been drinking at age 14, but very relieved that she'd had the sense to call for help, and it was ultimately a turning point where she began to be more honest and I think realised I was asking awkward questions about her plans because I am interested in her safety and wellbeing.