No, Jarse, I had not flown with the student prior to being asked, however, being the nice person that I am, I had always been pleasant/polite, said gidday etc. as everybody should do!
When he asked me....the message went to message bank, so I had a chance to inform the other instructor prior to returning the students call. It seems that they had a personality clash...both sides realised it, and I got blessings from both of them.
As a matter of fact, I actively encourage my students to fly with other instructors, so they CAN get a different perspective on things....of course I prewarn the instructor as to what he/she does/doesn't do correctly.
However, I normally only encourage them to fly with others when I am now available, or they are coming up to a check ride.