He is misinformed. Sounds like he may be trying to keep you from purchasing it for some reason.
You can discharge them to whatever level you wish and they DO NOT get a memory, that is what is great about them. Its best to leave them sitting at 70-80% charge for long times, but it is not necessary by any means.
You will need a battery charger that has a lithium or AGM mode on it, make sure you use the correct charger.
So to be clear, there is no discharge point and no required level for discharge. You can use it for 3 hours and be down to 95% charge and then recharge when you get home if you want.