If she is birthing it, tying might be redundant. I'd be a bit concerned about the ties with a newborn. I definitely wouldn't use yarn because of the choking hazard (even with older children). If she really feels the need to tie it, I'd go with short strands of floss.
I've made newborn blankets using solid cotton on one side, minkee backing and pre-packaged satin bias tape sewn on with a zig-zag stitch. I didn't know any better & used a universal needle back then. It worked great. Minkee is already plenty warm so I wouldn't recommend adding additional batting.