There is bobbin 60-weight thread available both on pre-wound bobbins and in spools for general machine embroidery. I use the pre-wound almost exclusively. They are available in a color range and of course black and white. Here is a link to a site I have used in the past. I find sales and watch for free shipping, etc.
http://www.allbrands.com/categories/...-polyester-fil
Be aware that different machines use different bobbins! My Viking Diamond uses "L" and I can use plastic sides, cardboard sides or the "sideless" type, but I don't care for those. They tend to "unravel" when I remove them if they are not empty (IMHO). The lighter weight bobbin thread is less expensive than regular "pretty" thread, takes up less room in the finished design, so isn't as bulky.
She can, of course, use the same thread (30 or 40 weight) and even the same color if the back would be seen (lots of changing out bobbins) and that is done lots with free standing lace items. I don't care for using cotton sewing thread unless I am after a particular look for a special design, say homespun, no sheen, etc.