If you just accidently caught the momma cat and had her spayed via a charity, how much would it cost ?

I might be able to find a donation towards treatment, to prevent another unwanted litter in the world, if you could arrange it.

If the owner doesn't understand that her cat is on heat , she'll probually not question if she goes off it too.

As for a shaved area on her cat, well even if she notices, the deed will be done, just deny all knowledge.

Let me know, if you want to try.