Without being gear specific, shoot an entire wedding from pre-wedding to actual day event using mirrorless with manual focus lenses or mirrorless with fixed focal length lens will be hard for me to accept. Some part of a wedding with some mirrorless system with known qualities can be accepted.
My remaining 2 questions would be the photographer offering to use mirrorless.
1) Is he trying to enter the profession with a mirrorless or is already a mirrorless convert from DSLR professional?
2) For the latter in Q1. Is he trying to trim his cost or feel that his deliverables with mirrorless is on par or better than with DSLR?
I am sure there are other clients/potential clients who think and feel like me. So I am really being the advocate's devil here to the wedding photographer's community who want to go lighter and probably cheaper on gears. Please excuse me if I have stepped on anyone's toe.