People suggested many different possibilities, but eventually several commenters identified it as an ab exerciser. The device is designed to strengthen the core through controlled movements while holding the handles.
Depending on how it is used, it can also engage the obliques, lower back, shoulders, and other stabilizing muscles. Its curved shape provides support during exercises, while its compact size makes it easy to store.
When he finally asked his girlfriend, she laughed and confirmed that she had bought it during a fitness phase but stopped using it after a few weeks. The mysterious object turned out to be nothing more than forgotten workout equipment.
