We saw something similar the other day. Aircraft ahead cleared VFR, as requested by pilot, to depart, we were cleared to depart with one ahead in sight. It wasn't in sight having just disappeared into/behind a cloud so had to wait for sufficient separation (we were IFR). No drama, just an incredulous comment across cockpit about VFR. Thank goodness for radar. But in this case how hard would it have been to call up for IFR (assuming the pilot was qualified and not going airways) - quick flight plan passed over the radio and off you go.