This is something that the 2 boys should have discussed at the biginning of the year. If both agreed on it then it would have been appropriate. If the other kid never "coached" your kid then maybe your kid should have taken the hint. Regardless, discussing the position with each other is not a bad idea. 90% of teams have no goalie coach or a coach that has a clue about goaltending (most will admit as much). I have always encouraged the kids to "talk" to each other about the position. What they find success in doing, maybe notice something the kid is doing different and why, etc. It is not appropriate to criticize every goal the kid gave up and what he should have done differently. That is not being a good teammate to your partner. Never discuss this stuff during the game. I want my goalies concentrating on the next shot and not what they could have done on the last one. After the game is appropriate
All that being said, almost every goalie has a goalie coach or instructor they go to. Let those instructors do what they are being paid to do and coach the goalies. Talking to each other about goaltending is not a problem and can be constructive. Coaching or criticizing each other can and will be.