Hi and welcome,
Vibrating sensations that only occur when she bends her neck forward, chin to chest sound like the Lhermitte sign, which is a sudden transient electric-like shocks extending down the spine triggered by flexing the head forward. It isn't exclusive to MS, other spinal issues eg over curvature of the neck (texting, gaming etc), structural disc issues, compression etc are also associated with this too, I'd think it's too close a coincidence after her fall, for it not to be the most likely explanation.
Hope that helps.....JJ
BTW, the vibrating sensations only occur when she bends her neck forward, chin to chest.