The link Iph provided certainly warrants an intervention. When the hair gets that long and matted, it affects the mobility of the dog--and it can be quite painful. I also think the age of the owner should be taken into consideration--particularly if you're talking about an elderly person who may have some dementia or other cognitive issues, as well as a chronic disease that makes it difficult for the owner to properly groom the dog. I'd say something to the family first, and if that doesn't work, I'd go to the local humane society. I wouldn't or couldn't ignore it.