I'm not sure how much his recipe feeds, probably about 20.
The gumbo is for a 70th birthday party my coworker is giving for her mother. I don't have a clue how many men will be there. It will be from 2-5 and there will be a limited amount of other food.
If it's not during meal time, you could probably get away with just doubling his recipe. Not everyone is going to be eating a regular serving and some may not eat at all. However, if you want to be really safe, he could just triple his. I make a lot at once, using 20 quart pots. Gumbo doubles, triples etc... very well. I don't make separate rouxs unless the roux is too big for my large cast iron pot. Otherwise, I just loosely increase the roux, the stock etc... Not an exact science as far as exact measurements.
The good news is that if there are leftovers, they can be frozen.
Honestly, I really think doubling his would be plenty. I tend to overmake, though. I have a fear of running out of food for guests. I need treatment. I would triple his and go with it. You can keep some for yourself.