Yeah just go in and (whether you believe it or not) say these things:
1. I'm sorry, I didn't really know what I handed you. (You say this because while it admits fault, it is polite and nice)
2. I think the threading must be different because this happened(explain).
3. I guess I need a matching formula lockring, is that right?
you say this because 1. while you could say 'you screwed up and i handed you a part that you should have known wouldn't have worked' that is not going to make them want to help you. if they think the fault is theirs, they'll fix it unless they dislike you. if they think the fault isn't theirs, they won't fix it unless they like you. basically, whether or not they fix it for free is almost entirely dependant on whether or not they like you. it's better that they like you.
2. they need to know what happened and you should REFRAIN from assigning fault. say "the lockring didn't fit the hub and fell off when I mashed the pedals." DO NOT say "the lockring didn't fit and fell off because you didn't install it right or notice it wasn't a proper thread combination."
3. if you give them reason to believe you'll be a return customer, they'll want to treat you well. if you come in angry or belittle or insult their service or expertise, they have no reason to think you'll come back and therefore no reason to help you out.
fortunately for me, i haven't been in your situation- all the work i've had done(very little, i didn't built my conversion i found it at a LBS) has come off without a hitch and i've been happy with it. i have great LBS(plural) in my area and everyone who i've dealt with are very smart, helpful, and personable salespeople. I treat them all with respect and they always go out of their way to give me good service and good attitude. I even like to think of some of them as friends.
the bottom line is, if you go in and don't address guilt or fault and are respectful, nice, and friendly, you have a far greater chance of them helping you out. if they're a good shop, they'll help you out even if you plainly say "hey it broke but it wasn't your fault, i gave you a wrong component" and they'll fix it for free because they want customers who are easy to deal with. however, if you ask them to fix it, not if they can fix it... they may charge you for the work and go ahead and do it.
did it strip your hub, or just the lockring? or is nothing stripped and it just fell out?
if ****'s stripped, i doubt they'll replace your 80 dollar hub even if you are nice and they think it is their fault. if it's just the work that needs to be redone i'd be extremely surprised if they don't fix it for you on the house. unless you're mean about it or accuse them of not knowing enough or doing bad work, in which case i'd be surprised if they don't turn you down the street to the next shop with no refund.