How old is the female? We can argue about possible repurcussions all day, but if shes that size and, say, five years old then it doesn't really matter. If she is only about a year and a half (Im guessing, that's about the size of my girls who are that age) I take the better safe than sorry route. If there's a possibility you could damage the health of your snake, why would you take it? I've heard of females being bred at 1200 grams, and I have also heard that breeding them too soon can produce smaller clutches which persist throughout the females life.
On a different note, why on earth would he be trying to breed with a normal male? A much better route to take could be purchasing a fire male (just an example) and take the next few months to let the firefly grow up a bit more and make sure this new boy is all healthy and ready to breed. This way there's less risk and more valuable offspring (black eyed lucys!). Not that offspring value should be a driving factor, but it would certainly help offset the costs of starting up a breeding project.