I have a friend who has been diagnosed with legionnaires disease and is in the critical care department of a local hospital. is this a disease that is mandated to be reported to the (u.s.) cdc? will there be follow up on the source of the bacteria?

Yes and yes: Legionella is nationally reportable- the lab that did the testing or the physician will report the result to the local health department, which will be responsible for further notification. Typically, if the source is not known some investigation will occur. This may involve testing water sources and other environmental risks, and investigating related cases.

No : Usually there is follow up but often unless there is a large outbreak the source lies at home!

Should be: Yes - the lab will notify the cdc can be found on mmwr (see google) the source is often from water. It would make sense to have it checked out.