Obama , I feel thats cheating ! either say the truth fearlessly or dont say it ! I dont believe in playing the middle ground when it comes to references for marriage purposes. Its not backbiting , as long as you know you have the guts to face the individual who you are talking about and say the same things to their face.
As long as one is not accusing the other of wrong deeds , i think it should be very clearly said to the investigating family of what you know and they can then investigate in that direction.
I don't think it's cheating at all. My aunty a super-conservative religious woman asked me about girls that she knew I was acquainted with for her grandson. I knew that the girl was too "out-there" for her grandson because I knew specific acts that were inconsistent with her ghar ka maahol. Instead of giving her a list of the girls harkatein, I used a euphemism telling her that I believed that the girl was too modern for her family.
My mother always says larkiyon ki izzat bahot nazuk hoti hai - apnay moonh se bura nahin kehna chahiyey aur kisi aur ki beti ke baarey mein buraa kehney se pehla (whether it's the truth or not) ye yaad karo ke hamare ghar mein bhi betiyan hai.