He should start by determining the overall worth of the collection by having the collection appraised. He can contact the American Society of Appraisers for assistance. If he has an inventory of the pieces in the collection, great. If he has an inventory with pictures, even better. You've already mentioned that the collection has size but does it have depth, i.e. did he collect older pieces as well as modern? An appraiser should be able to look at an inventory and determine if an appraisal is warranted and worth the cost of the appraisal. And knowing the estimated value of the collection should help to keep the wolves away from your door.
You could turn to local hobby shops for advice but I would suggest caution if you go that route. Best to first follow the advice of someone in the ASA. Good luck with your sale.