If a person openly cheats on her/his SO, is that person violating trusts he/she has with others too?

Let me explain and clarify.
A person (let’s call that person Rene and the sex is not important) that is close to me has a relationship from four years (with Sam).
Rene still has feelings for an old flame (Arie) and recently they met again.
Rene cheats on Sam with Arie.
Several people know about this thing going on (mainly because Rene told some persons, who, in turn told others) and strangely enough don’t seem to be much bothered by it.
It bothers me slightly.
Sam, after all, isn’t aware, but we are.
I feel sorry for Sam, but also slightly pissed by Rene and a little pissed by all who are in the knowing.
But mainly by Rene.
Isn’t Sam’s behaviour a proof that he/she is an untrustworthy person, not only and mainly for Sam, but also for us?

Edit to correct Sam into Rene (thanks @Blackberry).