If this is an individual tournament, then no it is not a coach. Unsolicited advice from a spectator, even if they approach the table is not something you can control. Especially when that advice turns out to be about something that the spectator had no idea what they were talking about.
If they were actually down and going to shoot the wrong ball and someone said something and then they change balls they are shooting at, that is clearly a coach as you can't control unsolicited advice, but you can control if you take it, thus causing that unsolicited advice to become a coach.
Best thing to do if someone starts talking to you is to tell them to be quite as you are shooting and it can be misconstrued as a coach.
Team tournament and your captain is in control. If they mess up, it is on them. From a sportsmanship perspective, I would not hold the individual responsible for someone else making a mistake.
From
http://vnea.com/8-ball-rules.aspx
H6. Coaching is a foul. Any member of a team called for coaching will result in a foul on the team member shooting.