I'm pretty sure that she's stopping right before her climax. I haven't had sex much, but I do masturbate relatively frequently.
That tenseness, and burning, is the last big climb. It can be a strange sensation, and even scary, to someone who doesn't know what to expect. However, that sensation is also the one that lets me know (and I'm sure many others!) that they're about to orgasm! It happens in seconds if the stimulation continues. I'll try to explain it, but it's difficult since it's a little different for everyone.
That tenseness is unbearable! Every muscles gets all tight, and I can see why people would stop thinking that they just can't go on. However, if you do, then you should orgasm, and your body feels like it instantly relaxes. You might twitch because of the pleasure, but light stimulation during an orgasm can feel amazing! It's hard to explain, but yes, continuing stimulation can feel great! (I'm surprised she ever stopped once she got to that point! haha) Good luck, and I suggest you explore your sexuality too. It helps you understand your body more, and can make sex more enjoyable whenever you do decide to take that step in a relationship, because you already know what you like!

Best of luck to you friend!