Biting and clawing is not related to having nothing better to do or being adopted in pairs. Those are whole other issues, but I'd like to encourage your friend to adopt a pair to have a playmate and for company. End of my 2 cents on that.
It's very natural for kittens to bite and claw when they're playing, whether with you (human) or others (cats). Some kittens do it more than others. My Kayla and Ginza never bit or clawed us when playing, but Mozart and Pixie were big biters when they were play-excited. For kittens like them, it's important to never use hands or feet when playing, so they won't see those as acceptable biting objects. Redirect their attention to an acceptable toy so they learn what can get bitten and "killed". If you teach them early, this is a behavior that can be stopped; otherwise, like anything else, the longer the behavior stays, the longer it takes to retrain.
Like Richo wrote, whether a play-bite hurts or breaks skin depends on the cat and how aggressive. When my Mozart forgets and nips me now, it's a very gentle one. But you can't really count on cats to be that considerate about it all the time. My friend's cat is out to draw blood when she nips.
You can find many good tips on this forum if you do a search, it's a common topic. Tell your friend good luck, and looking forward to hearing of her 2 kitties (hint, hint).