These threads often get diverted by strawmen and discussions about situations that bear absolutely no relevance to the one in the OP.
What also comes out is a rather one-dimensional view of why people are unfaithful and how they present in real-life i.e. that they are 'cheaters' and that this marks them out as distinct from people who aren't.
As some very wise posters have pointed out, the vast majority of people who are unfaithful to their partners do so because of sliding boundaries and a lack of self-awareness. Prior to an affair, the people who have them are often adamant that they would never cheat and so the consistently under-reinforce their boundaries with new friendships. They believe themselves to be unassailable and invulnerable to ego-boosts, attention and the addiction of new positive feelings about themselves.
So whereas a self-aware person who knows that a new friendship could be dangerous will avoid situations that cross boundaries, the person who is not self-aware and is in denial about an addiction to the feelings he's already getting about himself will continue to put himself in situations that test his resolve.
The people who are most at risk of this are individuals who are already a bit selfish in their relationships and who tell lies of omission to their partners.
So the OP's individual circumstances all point to her instincts being absolutely correct. Her husband ticks the selfish box, he is not being open about these nights out and he is not being honest with himself or the OP about how vulnerable he has been to getting addicted to the friendship and the positive feelings it has engendered in him about him.
The colleague however sounds as though she is far more self-aware, knows that this is dangerous territory and therefore keeps putting up some boundaries, despite constant temptation being put in her path by the OP's husband.