My friend lives alone but has two cats. They are house cats so don't go outside (she re-homed them from a charity several years ago and they cannot go outside for whatever reason). They use a litter tray.
She has had the cats several years, and I never used to notice a smell in her house, but the last few times I have visited her house really smells bad. I was invited round for dinner the other day and the smell was so strong it put me off wanting to eat there 
I assume the smell is caused by the cats/their litter tray as I cannot think what else it is. Her house is always very tidy and looks clean, the kitchen and bathroom are immaculate so I don't think she is a dirty person. My friend doesn't smell herself, she seems to have good personal hygiene and always wears clean clothes etc. so I can only assume the smell is caused by the cats and the fact she never opens the windows (for fear of the cats escaping) so the house never gets aired.
I would just like to add that I am an animal lover myself and have pets so I am quite used to animal smells.
I am not sure whether I should say anything? My friend is single and hoping to meet someone and I do worry that if she met a nice man the house smell might put them off.
Also I am quite surprised no one in her family has said anything (she has a few family members that live near her and visit her frequently) and I do wonder if I should just leave it to them and mind my own business?
Is there a subtle way of telling someone their house smells?
Thanks