Question about illegal(?) wiretapping

I have an acquaintance (seriously!) who recorded some conversations on his telephone line because he suspected some skulduggerous conversations going on (later confirmed). When he mentioned it to me, I told him I thought it was a bad idea, since that might be considered wiretapping. He said that “someone” (don’t know who) told him it was OK.

Now this is something that varies from state to state. In my own state, you can legally record a conversation that you’re a part of, but not one between two other people. However, the other people were using his phone line. In general, would this be considered an illegal wiretap? Is there any expectation of privacy using someone’s private line (or would that even matter)?

In most states, it’s an illegal wire tap unless the party on the other end of the line is informed that they are being recoreded.

did he plan on using the recorded information in the future (in court, for example) or was it just for his own personal use?