Don't Panic

Or more accurately when your vintage 50's silk dress comes back from the dry cleaners three hours before curtian looking like this;

PANIC!
and then step 2; stop Panicking and take apart the seams so that you can lie what is left of the fabric flat.
Sometimes these things happen, sometimes fabric that looks perfectly sound will disintegrate when confronted with dry cleaning fluid... even pros miss these things on occasion. That said... upon seeing the destroyed sleeve I did have a mega-queasy feeling. This is Bad.
Step 3; gently lay a light weight fabric of a similar weight behind the torn fabric and pin down
Do Not,(Repeat) Do Not use fusible interfacing or stitch witchery... hand stitch with a single thread the patch to the back of the tatters...
Step 4; See all of this mess... I did have to trim some of it away (sob)
Step 5; After the patch is hand stitched place the fabric on a good sewing machine and use a straight stitch back and forth, back and forth without overdoing it...
Step 6; reassemble sleeve and give it a good press with steam.
Step 7; HAND WASH ONLY from now on (holy crap that was scary)