Asking a foreign professor to be your thesis co-advisor I am a master's student and I started searching for someone to be my thesis advisor. I went to ask a well-known professor from my university, that I wanted to work with, and he was happy to help.
However, he said that his and my area of interest at the moment don't align. But, he did say that he has a good friend in the US who does exactly what I am interested in and has set up a meeting between us two.
My professor said that I should ask him whether he would be willing to co-advise my master's thesis however, I am not sure whats the best way to ask this question.
Since you clarified that the &quot;official topic&quot; is research and career progression, I would just let it come up naturally. After you have clarified your general interest in the topic and in doing a PhD., there is an ideal spot for something like:

&quot;But of course, before I can do that, I need to finish my master's here by writing a thesis. I think it would be useful if that already was in the same area. Professor X told me that he might be willing to be the official supervisor of this thesis as long as I can find a co-supervisor that matches the specific field better. He suggested asking you, so would you be interested?&quot;

You could do this in the meeting or already in the email setting up the date, though for the latter you should take care that it doesn't get to long, the details are what the meeting is for.