Cockatiel in poor conitions?

I went to my friends house yesterday and they have a Cockatiel. I didn't really notice the bird until i started getting ready to head home. I looked in the cage and she was laying in the corner on her back with her mouth open. I was freakin out, i thought she was dead but my friend said she always does that, and she's alive. He poked at the cage, and sure enough she got up and started 'hissing' at him. Her water dish had dry green stuff that looked like algae in it. And the food was hard and molded to the dish. Her cage was FULL of eggs.. and smelled really bad. He said it's his sisters bird, she didn't want it so she left her there, and since she's not his, he isn't going to take care of her. The bird is 10 years old, doesn't seem friendly what-so-ever. I asked him if i could at least clean the cage out and give her some new water and he told me to go for it. So i did. It took so long... But as soon as it was done, i gave her some water and she went crazy over it! About half an hour later she was sitting on her perch and chirping a little bit. I sat in a chair beside the cage and talked to her, and she didn't try attacking the cage or anything. My friend said she always tries to attack people when they come near her, he's never seen her open up and chirp at anyone like she did with me. He said if i want her, take her. Do you think it's a good idea? If i get her she definitely needs a bigger cage, cuttle bone, toys and grit... I do have experience with birds, i've got 2 Cockatiels and a pair of Doves. Definitely not expecting to keep this bird in with my other Tiel's, i just want to give her a good home and show her what it's like to be loved. Opinions? Thanks