Sorry, no grace on weight.  I once had something only gram or 2 over and they charged me the higher rate (it was a parcel not satchel).  Few grams in satchels thrown into mail box is something that you may get away with, 100g+ you would be taking big risk.  If the parcel gets picked up, there will be fine on top of the extra postage; sometimes they collect it from the recipient, sometimes they get back to the sender.

Your friend has 2 option:

1- let the auction end and send in 3kg satchel at his expense

2- cancel the auction and relist with the correct postage cost