depends on the gauge of her needles on hand. unless she was knitting tiny socks or a lace shawl they're not thin enough to get into a lock (certainly not a Yale lock like is the most common in the UK) and she's certainly not going to bend any of her needles to get the tapping action you need.
that's what i thought too, given you can knit a scarf from a single ball in a few hours. and i'm not even a good knitter (crochet ftw)
What i want to know is what kind of knitter can go into a yarn shop and only buy one skein!? that's some epic discipline right there; there's nothing worse than going away with two balls of yarn and running out then having to return to the shop trying to search for the right dye lot :'(
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