Yes it can be treated. You may need to have it put back and sutured. You may be able to use a damp cloth and push it back in yourself too.
This is something that needs to be taken care of right away as the tissue could dry out making it harder to repair it.
If you can't do anything today you might want to use a little mineral oil on it to keep it moist.
You can read some more on the problem here
http://www.bobmckee.com/Client%20Info/GI_Disease/Prolapse.html
You may find you will have to increase the fiber in your dog's diet to keep stools passing normally and prevent this happening again.
Hope this helps you!
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