There are folks using Dgraph for fraud detection and identity systems.
The main idea is that you can identify the same users as connected to (or not) related nodes in the graph. You can identify these users or devices by some ID and can start to understand the relationships via queries in Dgraph. Like you wrote, you can use queries to figure out which users connected to certain devices and furthermore what other things did those users/devices access.
Grapl uses Dgraph for their platform for intrusion detection. While not necessarily the same as fraud detection, I imagine there are similarities that may be interesting for you.