One issue I ran into has two parts. I think the first part isn't really questionable, while the second might be.
(1) I googled a friends name, and then their email. I've probably done this to 50% of the people I'm friends with.
(2) I asked a mutual friend that was in the know about some of the things I saw and we talked about it a bit.
(1) Just doesn't irk me at all. I don't feel any guilt for curiously looking at public information people have put out there among my friends. It's usually not that interesting. Even this time it was not that interesting, but the person involved is insecure about something... some of the time. Everything I post on the internet I'm willing to deal with. But I'm also a fairly... nominal... person. My weirdest bits would be things I've posted on this forum, and all those positions are relatively tenable.
(2) I could see being more of an issue, some claims of talking behind peoples backs, etc. I'll take some guilt and remorse there. But at the time I thought it was more innocuous, and if it was so readily available and public, I worked under the assumption the person was comfortable with that.