Dog smell,Old Dog Smell is pretty hard to deal with oily is worse, try a really thorough cleaning but inform customer you can only clean what you see not necessarrily what you smell..
If you want to try, remove as much hair and danders as possible, precondition with a prespray with a oxidizing booster (if the dyes are colorfast), extract as hot as your can, then apply an anti microbial suitable for upholstery fabric
I have had reasonably good success with Microban, Sterifab, and Bacitbarrier