PollyDolly
If the fabric is viscose is will easily fray as the weave is loose and the thread are 'heavy' for. want of a better word; viscose fabric hangs beautifully but can be a night mare to handle.
Personally, I would position a very small piece of iron-on interfacing to the wrong side, this will stabilise the fibres. then either machine or hand darn over the damaged part. If. you. don't have any lightweight iron on interfacing just PM me and I will send you a piece - save you buying a huge amount for such a small job.
Finally, once the repair is done, forget about it and enjoy wearing your beautiful dress.
Completely agree particularly in respect of the fraying capacity of viscose. The only thing is that the interfacing will need to be about twice the size of the hole just to stabilise the darning.
Sorry if this is obvious but I have known people to just fill the hole and not the surrounding weak area and end up with a much bigger hole.