If you're talking about letting them out to play together I don't think that would be too safe. Prairie dogs
can get very rough when they play and a guinea pig really can't defend itself. If you do decide to do it make sure you watch them closely and be prepared, with gloves, to intervene. I have pd's and guinea pigs and I have never allowed them together. I let the pd sniff the guinea pig through the cage and she went beserk, so I would never let mine out together.
Letting them live together would definitely be a no no since they each have their own special diet needs.
If you would like him to have a friend, look into getting another pd. I have 2 and they love to play and cuddle. He would probably love to have a friend just like him, plus it's so much fun to watch 2 pd's playing.
Hope this helps with your decision.