Outdoor cat becomes indoor cat

I visited my grandparent's house and they just got a cat. She had some behavioral issues (biting) and they said they had gotten her from outside. One time, we saw a cat outside the patio (it has windows and is completely enclosed by them) which we assumed she knew from when she lived outside. I opened the window and gave him a little bit of food and she tried to jump out the window to him. I caught her and closed the window, but then felt bad because it seemed like she wanted to escape. So I opened the window and held her and let her see outside, but she wouldn't let me take her away from the window. Now I'm wondering if it is cruel or not to keep her/keep her inside when she really wants to go outside.