I don't know if I could stop myself in order to wait for further evidence to arise. I would probably say something immediately to the proper outside authorities and let them do the investigation. Usually these types of people are doing more than just molesting children... they are keeping nasty files on their computer or doing something else. If they aren't then something else will be there, a parent's suspicion or something. These things aren't as quiet as people believe. I think the innocence of a child that is being molested/abused outweighs the individual's legal innocence and status.
It is a weighted decision. Nothing good can happen by being silent, but a lot of bad can happen if you speak up, but it still has to be done. I believe that if the suspicion is there and a source is present then that is enough to take action. If there is nothing there, then there is nothing there, but I would hate knowing that I knew or suspected and did nothing and a predator walks free. If the allegations are completely false and you spoke up, you lose the friendship. If the allegations are true you have saved a child from abuse. If the allegations are false and he is convicted of the crimes against him, then I would feel shitty about the legal system, but all I would be responsible for is telling the police about a possible child abuse relayed to me by a source that I thought was credible. I would be sure to express my concerns about that source if I thought they could be mistaken or malicious in intent. Would it bother me that I played a part in an innocent person being branded as a sex offender and going to prison, yes, it would bother me a lot.
All in all though I feel the investigation has to be made. So yes, I would do it. It is not an easy decision and I would not take it lightly.