My son's OT made him a heavy lap pillow for school. I don't know if it's full of sand or beans, but it's been very enjoyable for him, and he can lay it on his lap at a desk, which she might also enjoy. I had a heavy cotton quilt (think denim & corduroy& upholstery fabric) in highschool that I loved, and that might be another slightly heavy but breathable option.

It's my understanding that the 1/10 rule is referring to a blanket that would be spread out over the entire body so if she just wants it for a lap blanket or to put around her chest I agree, 16 lbs sounds like a lot.
I have made a few and found that one ounce if poly beads per 4 -5 inch square pocket seems to come out about right but do your own math on that. The blanket I made first is on the small side (probably about the size of a lap blanket) and it was 6 lbs. Its plenty weighty, I can imagine it being 10 lbs heavier.
Some other things you might want to keep in mind - weighted blankets get hot due to the poly beads so starting with a wool blanket might make it super hot. I've heard that glass beads don't retain so much heat and are still washable but they're more expensive. Don't use dried beans or corn or anything like that because if they get wet they can get gross.
If you have other questions, please feel free to ask.