Tell her and try and get her to be a willing participant.

Well of course you're going to tell her about it. It feels creepy to watch your friend orgasm without her knowing, and you feel that this will be more satisfying if it does work if she knows. You go over to your window, texting her as you go.
'Hey, got sumtin 2 sho u, com 2 windo'
It's only a few moments before she appears, still dressed in her school uniform.
"Yo, John, what's up?" Ana asks.
"Nothin' much," you reply. "I just got this gadget in the mail, and I wanted to try it out. I was hoping you could help me with that."
"Yeah? What kind of gadget then?" You hold up the little gun and she squints at it. "What is that?"
"It's the Orgasmotron 3000," you explain. "With it I can give anyone an 'instant, intense and satisfying' orgasm."
"What." She intones flatly. "How the hell is that supposed to work?"
"Dunno," you reply. "Some sciency bullshit. It says I just gotta put it on the desired settings, point and pull the trigger. Bam, instant orgasm." You grin.