That's a toughie. Buy a rain jacket, maybe?
I'm guessing the washing instructions say something like "hand wash, lie flat in shade to dry." The kinds of instructions I usually read and ignore because I don't hand wash anything.But I'm thinking that gentle hand washing might be the only way to go here....
Cut off the necklace and get Megan to adapt it so you can wear it again. Then wash shirt normally.
I'd chuck it in one of those 'laundry bags' for washing underwear (mine has a very tight mesh), throw it in the machine and hope for the best!(I've used this strategy a lot and have been surprised at how many things have survived.)
I was going to say much what Helen said but now that I've read Narelle's comment, I'd give that a try first. Worst that can happen is that you lose $7.95.