How to evict a squatter?

[Jonathan Johnson]

Hello BP,

I'm looking into buying an apartment complex from an elderly fellow. He has a "friend of the family" currently as the only "tenant" in his 6-unit apartment. He told me she has not paid rent in over 15 years, and he's even paid her utilities for a while. He describes her as not all there, and she has a lot of cats. 

I read the How to evict a tenant guide, but it doesn't say anything about someone just squatting with no lease or rent.

She is probably really comfy there with a lot of stuff. I'm going to offer her to sign a lease once renovations are done, but I'm 94% sure she won't go for it. 

How should I approach this? Treat her just like a tenant and start the eviction process? Any bumps you could foresee that I should be aware of?

Side questions: 

How do you get intense cat smell out? 
Can you write up a no-cats policy when the last owner didn't have it?