My Juki 2010 rarely does this and I use the thread cutter all the time. My Juki F600 however makes a thread ball 90% of the time no matter what I do so I keep a scrap handy if I'm not piecing with leader/enders. Does your friend take off her throat plate and clean out the lint regularly? I'm good about cleaning the bobbin area but sometimes let things accumulate under the throat plate too long.