@bluesky123,
Nothing.
You are no longer together. Who cares how she met this guy? Would you feel better or worse if she met him at the grocery store, or through a dating service. Or, gasp, a bar??!??!!?
The bottom line is, you only have cause for concern if there is the possibility of harm (physical or emotional) to your son.
Otherwise, you must let it go.
This is the "moving on" part of the process.