If your local law makes it illegal to withold an employment reference, will this do?:
"This is to certify that.........worked for this company in the capacity of........ from ....... to .......
During his/her* employment he/she* always/usually/occasionally* (*delete as applicable) met the minimum regulatory flight standards. He left on short notice/gave the required notice* (*as applicable).
As the person reading this reference is probably one of our competitors, we would thoroughly recommend that you hire Mr/Ms............."