Is it possible to obtain a replacement compestela?

It can be done, but it is easiest in person. I had a similar loss and obtained a replacement some years later when I was able to return to the office in SdC. I had tried by post, following the best instructions of @t2andreo but the office was so busy that I never did get a reply from them.
When I arrived however, it was easy for them to take my ID, look me up in the system... hear my story and grant me a replacement. The woman who wrote out the new one cried with me... hugged me tight... and I grew even more admiration for the people who have the strength to hear our stories on arrival to their offices.
If your friends cannot go, perhaps they have a friend who will be going who can try to get the document replaced?