Well I’d see it as a challenge. Firstly the shower is obviously a good start. Also sat on the toilet, seat up or down depending which you find more comfortable.
Then there’s trying it in public restrooms, some people love that, others find it understandably icky. I just encourage you to give it a go, you might love the thrill of doing it secretly, but somewhere public.
In which case it opens up a whole world of possibilities, rest rooms, changing rooms, etc.
Next, for even more public, but even more secret explore the (literally) hidden art of thigh masturbation – many discover they can edge just by crossing their thighs and squeezing rhythmically. Read more here:
https://female-orgasm-denial.tumblr.com/search/thigh+masturbation
Lastly, edging really isn’t a noisy activity. I’d suggest you try being brave, and when it’s dark, do it in bed! Decide it’s fun and naughty to do it silently without her knowing. If you lift your knees and spread them a little you don’t even get your hand rubbing on the sheet to make a noise. And what’s best is that it’s a HUGE disincentive to orgasm, because you probably would make a noise, so it’s actually great if you want to get better at denying yourself.
Beyond all that, if she’s a friend and you trust her, and your intuition tells you she might be kinky too, you could even tell her about denial. Check out all the denial buddy ideas, some of which are specifically designed to tell a friend about it and share the thrill of edging. See if you can lead her to this blog (hint, anyone just googling ‘female orgasm denial’ will find this!). You wouldn’t have to be quiet at all if she’s doing it at the same time, would you…
https://female-orgasm-denial.tumblr.com/tagged/denial-buddies/chrono