I was asked to be guarantor earlier this year in very similar circumstances. I telephoned the letting agent to determine exactly what the implications were for me, and why the potential tenant needed a guarantor.
I was told about his bad credit record which helped me make my decision.
It seems to be standard practice. After all, a guarantor is there to pay your rent if you don't pay, can be taken to court and end up with a county court judgement and a bad credit score himself, so it is only right the guarantor should fully understand the risk.
I would be furious to have these facts hidden from me, given that it is extremely difficult for an individual to get this sort of information himself.