Coworker too friendly?

So my husband has been working for our apartment complex for a year now in the office. They got a new coworker yesterday and he gave her his number for purposes like work related things. Now our marriage has been rocky dealing with marital issues (no not cheating). She texted him last night and said oh I don't mind working weekends if you want to switch. And also said have fun tomorrow because he's working today.(with an added smiley face) then continued to say sweet dreams. He told me what she said last night because she texted at 11pm but left out the sweet dream part. She's literally worked with him for a day and doesn't even know him. Am I over reacting or is this inappropriate? She knows we are married and its someone I used to go to school with and we have a baby on the way. What do you guys think? My hubby said that was flirty but she has a bubbly personality too and he left out the sweet dreams part because he didn't want me to feel insecure and freak out. I just don't want her to continue saying weird things like that.