Yes, it's dumb to flirt when you already have someone, no matter your intention.
I'd be concerned.
No but it would soon..
Some people are flirtatious by nature. This being said its fine if its been a constant through the years and a problem if its a recent occurrence
The person must feel some dissatisfaction with their present relationship or they wouldn't be flirting with someone else. Flirting once is one thing but ongoing is another. If you improve your relationship and bring more romanance and excitement to it, the flirting with the coworker will naturally stop.
Some couples are ok with that sort of thing. Some are not. <br />
If one person in the relationship is not ok with it, then it will present a problem.
Funny that 'this' person doesn't realise he/she has the most to lose... The other person probably doesn't give a ****...
Never **** where you eat. Messing with a work colleague is bad news. I would be concerned big time.
I don't think it's worth ending that kind of long standing relationship over, but the behavior is definitely NOT okay and they've got a lot of making up to do.