How to tell my boyfriend to be more assertive?

Sorry about the title it's a bit vague. My boyfriend and I have been together for almost a year now and we are so happy together just like the first day we met. Anyhow my boyfriend work for apple and this requires him to do a lot of travel. He has this coworker that is always with him and at first I didn't mind at all but when he comes home and tells me that his coworker tried to kiss him it makes me mad. They both have each others phone numbers and she constanly texts my boyfriend at night asking him to go out with her and have a drink or go to parties with her. My boyfriend is a good guy but he's not an assertive guy. We went to a company party and I saw her and she knows we are happily together but she just won't stop chasing after my boyfriend. My boyfriend doesn't text her back or engage her at all only if it's for work but he also hasn't told her to stop doing all these things. I never told him about how sad I get because I am afraid he'll think I am insecure about myself or that I dont trust him, I just want him to tell her to stop. Also for girls why would another female come and try to steal another women's man away what provokes them?
How to tell my boyfriend to be more assertive?
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