Was it fine for me to let my girlfriend make her own breathing equipment?
I really don't think that is a good idea! What did she do?
She's really do-it-yourself, and wanted to go hands on.
She's inexperienced and new at scuba diving. She really should talk to a scuba diving expert before buying and using any equipment.
She already made the equipment and is ready to use it, so I think she can handle it.
No one starts off an expert, so give her a tutorial and answer any questions she may about the breathing apparatus. You should care about her safety and take the necessary precautions so she is safe while underwater.