The bone graft is not necessary unless you expect to have implants in the future. If she feels the graft is needed for the partial then of course it would have to be done, but that is usually only needed in extreme cases.
Is there is severe resorption of the ridge under the bridge area, a bone graft may help rebuild the area a bit so that it supports a partial denture better. Some razor edged, thin ridges can be uncomfortable when biting forces from a partial denture are applied on them.
You might consult your dentist who is making the partial denture for you to give you their opinion about how things would be without the bone graft.