

[Advice] Gf keeps her dog 23 hours a day in cage
Advices needed since I'd never had a dog before. My new gf keeps her 5yo bull terrier in a cage all day except for the two 10-minute walks that she takes him every day. Is this a normal and responsible owner behaviour? I always feel bad for the dog but I don't know if I am overreacting. For context she lives in an averaged-sized flat/apartment, so letting the dog out in the house doesn't exactly give him a whole lot of roaming space, but it's still better than being locked up all day I feel? Whenever I bring this up, she'd say the dog was brought up this way and has long seen his cage as a home/safe harbour so he's fine with it, but I'm not so convinced. I'm concerned that the dog is not getting enough freedom/mental stimulants and I'm looking for any feedbacks and comments on this. Thanks in advance!
Edit: We live in the UK if it matters
Edit 2: Wow I did not expect this post would get this many responses. Thank you all so much for pitching in, I hear you loud and clear - this is unacceptable and something that needs to be dealt with immediately. I will speak with her sometime this week (I reckon I should do this in person) and make her understand. I just hope no long-term damages have already been done; the dog is otherwise physically healthy.
I will perhaps update this post with a follow up. Thank you all again (and sorry for not being able to response to each of your comment - they are very much appreciated tho)