His girlfriend is jealous of me!!!!!

Hi! I have an ex- collegue at work and we had a great relationship as friends... I sometimes felt like there is something between us but ignored it... I didn't know he had a girlfriend.. He never told me and never acted like it... One day he was supposed to call me to see each other and talk about work... He didn't call but finally texted me and told me that his girlfriend is jealous !!! He told me he cannot speak at home because of it and that he feels more comfortable talking at the office. I was shocked!!! I mean, why would he tell me this??? If he has a girlfriend he should know how to act and what are his limits... I felt bad about it.. He later told me it was a joke and that i should not take things so seriously... I just don't understand... Please tell me your opinion...