Tournament rules

Is it ok for me to send a message to another tournament participant advising him how to beat a player we both face?

If it is an ongoing game then no.  That is considered cheating.  

If the game hasnt started yet, and yu want to offer some advice, than yep thats fine.

Thank you. What would happen to me if I had done this unknowingly? 

Well if you didnt know, then you didnt know.  But common sense, and the sites rules that you read when you joined both say its cheating.  You wouldnt offer advice to someone playing a rated game at a OTB tournment?  Its no different here.

I would just say: "Do your best, you can win! Good luck!" But I would never advice anyone about what move they should make to beat someone....

On the other hand, sometimes I may encourage my opponents by giving them compliments for good play, or telling them when they make mistakes, or call it a fork when I fork them. 😄 if it can help them to improve or keep up.

If you're giving advice to a weaker player or one who lost a couple games and won't advance or win a tournament for the purpose of beating a higher scoring player so that you'll have an edge in total scoring against the higher scoring player, it is ethically wrong even though there's no strict rule against it.

 I agree that is wrong. I had someone advice me on games against other player before, and I decided to ignore their advices. For me it was just wrong to ad a better player as my "team mate" against another player. Chess should after all be a single player vs another single player.