I am sorry that your client was scared into a scheduled c/section. The exact same thing happened to me in October 2005. My baby was 9lb8.5oz.
And yes, it is standard protocol in hospitals to check the glucose of babies over 4000g. There may be some evidence somewhere that large babies have troubles with their glucose levels after birth. I think the reasoning is that "Oh, we must have missed GD, the baby will have problems controlling their glucose now." My dd was tested, and her blood glucose was just fine... every single time. I nursed her like crazy in the hospital.
I am sort of curious, are you continuing to support your client? Did you attend her c/section still? My doula sent me some info from ACOG on not inducing or scheduling a c/section for big babies (unfortunately I wasn't in a state of mind to use it to argue with the doctor). She didn't offer to come to the appt. where I was talked into the section - I was too afraid to ask, and after it was decided she gave me a partial refund, did not attend my "birth", and she didn't do the post partum visit. I did go and visit her myself later so that she could meet my daughter. I felt kind of abandoned, and wondered if this was typical of the situation.
Good luck to your client, I hope that she has a better time healing from her c-section (mentally) than I did.