Yes, this is a bit confusing.
It sounds like he substituted 6 pounds of DRY malt extract for
6 pounds of LIQUID extract and 1 pound of Dry extract.
Which should be right, since normally you need more liquid than dry. Most cans of liquid malt extract (LME) are 3.3 lbs and can be sub'ed for a 3 pound bag of Dry Malt extract (DME)
I think the store was correct, and I'd just brew everything normally. Have fun! Let us know how it turns out, whatever you did.