Be sweet, be kind, touch her, compliment her, tell her how much it turns you on to have her like this, but whatever you do, keep saying ‘no’ if she asks to cum.
Say it sweetly, say it kindly, say it as you rub her, but just keep saying ‘no’.
As she gets hornier and hornier, if you ask the right questions she’ll start to open up in ways you can’t believe. Caress her as you ask her to tell you the real fantasies she has, the ones she used to masturbate to, but never told anyone.
Speak them back to her, elaborate on them, make them your own, suddenly even her darkest desires are out of her control. And if there’s things you can do, safely, consensually, then gently push to see which of those things she’d like to actually explore (some are just fantasies and that’s totally okay)
And cover it all with a clear agreement on safewords, so if it does become too much, if her begging to cum becomes more than she can stand. She can tell you.
But unless she does, just keep saying ‘no’ – you’ll be amazed where it’ll take you both.