Going blind is no picnic for a horse. I will be brutally honest...be prepared for the worst. She may panic as her sight digresses; if she is in with other horses, that CAN help, but it is not a 100% given that she will rely on the other horses to help guide her. My mom's arab has been progressively losing his sight through out the winter, and he is not adjusting really well, even though he has a buddy; he panics alot, bumps into things, and just has a hard time over all.
Make sure the area she is in is really safe; so, if she is in an area that there are stumps, trees, etc...you may have to consider moving her to a different paddock that doesn't have all of those obstacles. I would want a horse that is losing sight in a panel type pen...where if he bumped into the panel, it wouldn't break, nor could he really get tangled in it... Finding water will be probably one of the most difficult things for her to find, at first when she loses her sight; she may get pretty scared, so you may have to be with her for a while to ensure she is able to figure out her trough, or buckets, so she doesn't dehydrate.
Safety is the key, I think, here...as long as you can get her in an area (hopefully with a horse that she really likes-and yes, put a bell on it!) that she can't get tangled in a fence, or run into sticks, stumps, and other natural 'dangers' to a blind horse, you may be able to get her to adjust. But it will take time...
As far as training, in regards to riding, or other handling...she shouldn't forget anything...she will just need some major time to adjust and regain confidence in herself and her surroundings.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."