It's not suitable for a quilt. If she insists then make one and let her worry about keeping it together.

I opened this thread with thoughts of 'ooohs' and 'ahhhhs' then I read "quilt for a 4 year old".

Broke my heart that she would consider using such a treasure for a 4 year old boy to drag around the house.

Aside from my personal feelings ... I don't think the fabric is well suited to further use. I would encourage her to find a way to have it professionally preserved.

This fabric (which I cannot see) is too precious as is - should be preserved and not touched!

I would not make a child's quilt out of that precious wedding gown. It should be used in a wall hanging after taking it to someone to get rid of the mold. Put the wallhaning behind glass so the child has it for rememberance of grandma. make one of similar colors with some embroidery(copycat). something sturdy for a child to drag around and get washed many times.

The part that is water damaged appears to be made of a different fabric,maybe cotton? If it is, can that part just be removed and not used?
As for the other, the silks are beautiful and look very fragile, so I think I would reinforce by hand the parts I could and then I would use an interfacing on the fabric before I cut it to make the quilt. It looks like it would make a beautiful crazy quilt, but I sure wouldn't give it to a child!!

I can understand her wanting to make something special with this gown but not a quilt for a 5 year old. Most silk can't be washed and never something that is as old at this. I would never take on something like this. I would go crazy of something happened while I was working with it and there would be no way to replace it. This is too much for someone to ask as a favor.

Can you go to a fabric/quilt museum and ask the staff?
The sentiment is nice but older silk sometimes shatters.

Maybe compromise a bit. I would reinforce one of the collar pieces with an iron on stabilizer and incorporate it into the child's quilt. But the majority of the dress should be preserved and used for another purpose. A wall hanging would be wonderful.

This is what I was thinking -
Silks generally aren't washable -- I don't use anything that isn't able to go in my washing machine, and I'm surely not going to give anything unwashable to my kids!

Oh what ashame that lovely silk should have been left alone wraped properly to stablize it and held for a keepsake in a treasured place or framed , a quilt for a 4 yr old no no no.