what can cause diarrhea in dogs?

[mumineedapooooo]

a friend is worried about her dog.she says it has been messing in the house every night for the past 2 weeks or so and noticed for the last 5 days it has had diarrhea,
shes going to the vets in the morning and asked whether i know what it could be,thought id ask on here for advice.
she says the dog hasnt eaten anything strange or a different food, which i know can disagree with some dogs (mine doesnt get on with chicken so we avoid giving it to her)
any ideas?

lots of things, just like people. I had a rottie that pretty much had soft poo for his entire life, it didn't matter what we fed him on. But if this dog has suddenly become incontinent and now has diarreoh then it needs investigating. How old is the dog and what do they feed him on?

the dog is quite old,shes had it since she was in her middle teens and is now 27.she has biscuits from the vets and tinned meat i think,ive txted her but not replied yet