Blood in his poo.

i take this little terrier type dog out, (my neighbours) and 2 nights ago he had a bit of a runny bot. and his owner said he sometimes ate what she eats. he begs she gives. so i said that alot of human food isnt good for dogs. and anyway tonight ive taken him for his walk and noticed he had blood in his poo. he was then trying to go again. and all he passed was like muscus blood. so i said to her about it, she said he had been licking his bottom today. so i advised her to ring the vets up. she did say she wil see what his poo is like in the morning. hope the ole fellas going to be ok. hes a contankerous dog. just wondering what it could be?? he dont go off lead, so cant of eaten anything bad for him.
any ideas??

Was it bright or dark?
Lighter red blood indicates that the bleed is near the end of the digestive system, e.g. if he's been constipated and burst a few blood vessels trying to "go".
Dark red/brown/black blood is more serious and can indicate internal bleeding so a vet trip would be a priority.
Mucus in the poo can be signs of a bacterial infection (this doesn't necessarily have to have been contracted by him eating human food) or a few other conditions
http://www.petmd.com/dog/conditions/...mucus_in_stool