Why do some women/men ask their partners not to talk to other people?

My friend told me he’s not allowed to talk to me anymore as per his new girlfriend. I don’t understand? I’m married with one child on the way. I also work with him so I need to talk with him in some capacity but she’s made it clear we can’t chat outside of work. I was actually looking forward to doing things as couples together. Like dinners, events etc. Any advice?
Why do some women/men ask their partners not to talk to other people?
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