I'm not sure your statement is correct. As long as the person recording the conversation is also actively participating in the conversation, then the recording is legal.
I'm not a lawyer, but as the recording was obtained legally, I don't why it would not be admissible in court... similar to evidence submitting from home security camera etc.
@Fatwa I guess we can consider this a gray area? There are several acts and laws that overlap and cover this. From PIPEDA to Criminal Code to Radiocommunication Act. Technically it is legal in certain situations. Others not so like what @tallman stated regarding businesses.
