
No need to be ashamed if you get straight onto it and do the moral and honourable thing. Get her to the vet and get the mismate injection. Plain and simple no need for worry. Some vets prefer not to do it till 2-3 days after the mating so you will be fine. :-)
I am guessing you have had a male and female "pairing" before when you say you are always careful a few times , if the dog is only 11 months old then at the bitches previous season he probably wasn't able to mate her anyway as he was a baby. This has to be the first season that he could have caught her so your first experience at keeping " these" pair apart.
I wouldn't let this litter progress even if it was a suitable mating. All bitches have a chance of requiring a c-section when having pups if you bitch is allergic to anesthesia she could possible die without assistance. To me this would be the most unethical decision of the pair and most risky.