You say you have a great wonderful relationship and that you are both happy! The problem is not him he may well be wanting to furthur pursue your girlfriend, but you should know by now whether you can trust her or not. If you have a great relationship then you would be sercure enough to know that she will not do anything and is just going for coffee with a guy from work to have a chat, were women we like to chat and dont think like guys.

Relax she told you about the guy and she thinks it innocent, you need to chill and respect and trust her enough to let her go out with male and female friends she will either learn that this guy has other motives or make a new friend. If you trust her and respect her and show her this by how you react to her choices it will only improve your relationship. If you start stypid petty fights then you will only lose her and your relationship will never last.

learn to communicate effectively if you can, jeolousy is never a good sign of a great realtionship as it represents that you dont trust your partner.