when to be quiet

i am known in my circle as the person who is going to tell you.  this means that there is an unspoken rule, 'don't ask me, if you don't want to know'.  now i am not cruel and i do not necessarily express my opinion if i am not asked for it but when i am asked when do i remain silent?  a recently divorced girl friend asked me about a guy she met online and i didn't hesitate to tell her, this particular person was a bad idea.  now don't get me wrong, i am sure he is nice, but i am a firm believer that if you divorce someone there is a reason and you shouldn't start dating his clone (looked similar with similar personality traits).  so i told her.  but i have another girlfriend who is currently experiencing a medical crisis during a pregnancy and i truly love J, but when asked my opinion i kept it to myself.  i felt that this situation it wasn't my place to influence.  so my question, did i let her down?  i hope not, but all the same, somethings are just not your business.