i can't stand going to his mothers house anymore?

I've been with my partner now for 3 and a half years. he's 24, I'm 22. he moved out alone last year and i live with my mother. the first time I went to his mother house the first thing that hit me was the smell. it reeks. She smokes inside the house(she's renting, also me and bf are non-smokers) right next to the clean clothes that are drying on the clothes horse. she has a very neglected dog and a clearly malnourished cat that live inside all the time(i can't report because I'm the only one who see's that its a problem and don't want to cause issues in my relationship), the dog goes to the bathroom anywhere it pleases. I've been begging them to take the poor dog to the vet for 3 years because it has a huge lump on its side (chihuahua with a baseball sized lump). I hate the way my clothes reek when i leave her house after only an hour. whenever i stay the night there my hair turns greasy, even if I've just washed it the night before. I used to spend the time in our bedroom because when we weren't there she would leave the door closed so the air was fresher in there, the carpet wasn't as putrid as the rest of the house and the animals never came in. now she leaves the door open and lets the cat sleep on the bed and the dog sleep on the floor. We've been going there less frequently because my partner is sick of her asking for money when his sister who lives with her just bought a new car. but last Friday night we went to visit as it had been a month. i got there late trying to spend as little time there as possible, but within 15 minutes i got up to get my handbag so i could go shopping just to get out, right as i slept into our room i kicked a piece of dog poop. i almost vomited but managed to hold it back (not that i think it would cause much more damage to the carpet). i left and didn't come back for 3 hours, when i came back i sat on the couch and the dog came up and peed a foot away from me. she threw a towel on it and left the towel there overnight to soak it up, the next morning when i got up there was another poop right next to the towel. she never cleans anything except clothes and dishes.

another issue is that I was hospitalized on the 30th of October 2010 from drinking water at her house, it gave me the worst stomach pains I've ever had, and for months afterward i had to keep a strict diet until i recovered because of the damage it caused to my stomach lining. (they won't believe that it's the water, even though i only get the horrible stomach pains when i drink their water, and they all have IBS, which my partner mysteriously stopped getting after he moved out).

His birthday is coming up the weekend after next and he will want to stay there in order to see his mum, i don't want to stay there, but want to spend the time with him as we only see each other on weekends(we work far apart) and it will be his birthday after all. should i invite her to his apartment instead or is that just prolonging the inevitable talk i have to have with him?

How do you tell someone you can't stand their mothers house? does this make me a horrible person?