I've seen worse at the Vet clinic I worked at. Porcupines can be a great play toy for most dogs
3 of my dogs have been in a play session with a porcupine...one of my Newf's I had on a leash walking along a trial at dusk. She stuck her head in a bush and came out with her mouth and nose, chin covered in quills. My Rottweiler came across one in my yard and had quills all through her mouth, face legs, even back legs...looked a bit worse than the Bull Terrier they show on the link. Luckiely my dogs are easy to work on and I removed all quills even between the teeth, back of mouth, tongue with no problems.
It happens to the best of owners...it can happen very easeily, even on lead if your not paying attention and checking every bush your dog pops its head into.