I recommend that he purchase some Panasonic DECT cordless phones with talking caller id. Program the Pana directory with the names and numbers of your frequent callers. When your friends and family call, you will here their name announced and know to answer the call.
note: Calls from Cell phones are usually ID themselves by # number only, but Pana phones will announce the preprogrammed name for the number.
If you want to take a step further, port your land line number to an Andriod smart cell phone and bluetooth to Pana's.. Smart phones can block a nearly unlimited amount of telephone numbers.
When the scammers call on a blocked number the cell phone pick's up on the the first ring and dumps them into voice mail(which you leave un-setup).. Your phones won't even ring, and it won't cost you anything, but it will cost the scammers a connection fee. Without the possibility of of any productive results, this will steadily decrease the number of rouge callers over time.