My partners boss -.-

Me and my partner have been together for nearly 2 years and he did a cover shift in another shop and then that night a girl he did the shift with started messaging him asking how his day was and stuff, I let it go until she sent him this face: 😘 which to me is flirting! Full on no denying and now guess who has just been promoted to his manager! And I’m not going to lie I hate her! Just the way she smiles at him pisses me off, the fact she thinks that’s okay! Like it’s not and now because he now works with her day in day out he’s defending her saying she wasn’t flirting, am I crazy or should he understand my feelings are valid ? Xxx