As someone with ADHD who loses things chronically, I have to say that I lose important items even if I’m the one paying for them. Every time I buy a new phone (5 in the last 1.5 years, BTW) I tell myself this will be the last time I am so careless, because it’s expensive, wasteful, and so embarrassing. I don’t lose things because I don’t care about them or underestimate their value, I lose things because I am basically indiscriminately inattentive, no matter how important something is. Medication has really helped me be more consistent and organized with where I put things, and the Tile app and tags help me when I misplace things around the house. However, neither of those are fail safe, so having one spot where I put my important items (a small filing cabinet for documents, a spot near the door for keys and wallet) is really helpful. I also recently started using a Thread wallet, which means I can keep my important cards and cash on a keychain with my keys. For whatever reason, having my keys and my wallet attached to each other has significantly decreased how often they get lost.
I’d be willing to bet that losing important things so often is really stressful and embarrassing for your girlfriend, and I’d also be willing to bet that your support and interest in helping her means the world to her. I know that’s the case for me with my husband. Good luck to both of you!