My Sister Is Experiencing Sudden "lightning Bolt" Flashes Of Light

My sister is experiencing sudden "lightning bolt" flashes of light in one eye, beginning in the upper outside corner and going across vision field. Started gradually in late afternoon and is worsening. Your web site seemed to indicate immediate medical attn. is needed. On Saturday evening, it's hard to come by. What harm if she waits until Monday, assuming it could be retinal detachment?
Thanks.
Linda