The client is using Group Pricing (per Item) https://www.zen-cart.com/downloads.php?do=file&id=154

If you look at my previous reply, you will see how I modified the function in sfl_functions.php to look up the group for the logged in user, and apply the correct group pricing.
As this is a mod to make SFL work with another mod, if you wish to incorporate the changes, maybe have it check for existence of those fields before allowing the additional code to run?