I'm thinking of deleting my girlfriend's saved games because she's spending to much time on them.
That sounds pretty controlling.
She was quite frustrated when I asked her to save her game and quit playing
You're right that your girlfriend is probably playing too much, and that this is bad.  However, it is worse to go behind her back and knowingly delete her progress.  This will just make her angry with you, and not do anything to help her addiction.
It's not like I'm wrong. Also, she will never know what happened.
That she might not catch you doesn't make it OK. Maybe she does need to back away from the games, but you're betraying her trust by hacking into her account. Your action would only be destructive without addressing the issue; it would be way more positive to talk to her about it.