It's legal if one side of the conversation is aware.

Both sides need to be aware. In some instances a verbal mention of a
recording must be announced. In others, an intermittent audible tone or beep
is sufficient to constantly remind both parties that a recording is being
made.

Which of the above is accurate depends on what state you are in.
In some states, the recording is OK if one of the parties is doing
the recording (and therefore knows about it). In others, it's not.
ALL parties have to know.

In no state (and I believe this is also a federal law) is it legal
for the husband to monitor the conversation between his wife and
her lover (or, in the case referred to, for a mother to monitor the
conversation between her child and his friends).

Gordon L. Burditt