Could you share a picture of the crack? Impossible to give any kind of clue without having a feel for how severe the crack actually is.
In general, I refuse to string any frame with a crack that’s going deeper than the paint. There is just no way to make an educted guess about how much a damaged frame can take or can’t take. If the owner insists to have it strung anyways, then go as low as possible and make sure that the owner knows that he will have to pay for the job even if the frame completely dies during the process. And be careful not to get hurt during the job.