I work as an Australian lawyer but I live in Israel. I have many Jewish clients in Australia. If they are not religious, I am careful not to send them an email on Friday (when it is already Shabbat in Australia) becuase I am concerned that they may read it and I would be guilty of lifnei iver.
I once received an urgent email from a non-Jewish client on Friday my time (while it was Shabbat in Australia) and I needed to respond. I asked a local Rav if it was OK and he said that in a case of need it is OK to send the email since the recipient is not Jewish and no-one else would see that I sent the email.
I now have a new client who has lived his whole life as a practicing Christian. He knows that I am Jewish. On a recent zoom meeting he proudly showed me that he was wearing a magen david and he told me that his mother told him that he was actually Jewish but he has no idea what that means. The next Friday would you believe it, he sent me an urgent email! I was reluctant to reply because even though he lives as a Christian, there is a pretty good chance that he is Jewish. Can I send him an email in these circumstances when it is Shabbat in Australia?
Life is full of nisyonos, and of course the week after finding out this information, it will cause you a significant challenge. If the person is Jewish, then sending him the e-mail, will cause him to be mechalel Shabbos over there. What you can do is send him a delayed e-mail, so that he will receive it only after it is no longer Shabbos for him. He won’t get his answer right away, but it is the best that you can do.