A female colleague sent my private message to her boyfriend by mistake. How can I reply without embarrassment?

I think if it were me, I wouldn't be embarrassed if others sent me the wrong message. I think it's the person who sent the wrong message. Sorry. You can relieve each other's embarrassment in the following ways.

First, reply to her humorously in her boyfriend's tone, but let her find out that she sent the wrong message, that is, tell her who you are indirectly with your language art. If it is a short message like a daily conversation, it will be easier to answer. But if it is an ambiguous message, if you are close, you can also reply to her humorously, and reply to her message in a way that relieves her embarrassment, which may also deepen your close relationship. If your relationship is not so good or you are not the kind of friends who talk about everything, you should carefully consider how to reply. For example, you can ask her with a puzzled face what happened, whether she was playing a prank, or whether a child touched her mobile phone mischievously.

Text messages. This way can solve each other's embarrassment well.

Second, tell her it's not an embarrassing thing. For example, talk to her about some experiences of sending wrong messages, and even make up some experiences. It's the same kind of text message sent to my boyfriend. The purpose is to make her feel some similarities between you, and then make her feel that her behavior is not so wonderful, which will effectively alleviate the embarrassment.

Finally, maybe she doesn't feel so embarrassed, but this kind of thing is embarrassing in your own cognition. Generally speaking, don't be so entangled in how to reply to her. After all, you shouldn't be the one who is embarrassed.