You should find some authentic Cajun recipes. We *love* crawfish and have an endless amount of recipes for them. I've never had saltwater varieties of crayfish, but I would assume they taste the same or very similar.
You just have to be careful not to overcook them. They cook quickly and become chewy when overdone, similar to shrimp.
If he can get some Zatarain's Liquid Shrimp & Crab Boil, he might like it. It tastes excellent with crawfish. Start off light, because it will be really hot if you use too much.
I recommend he try them in a good etouffee, jambalaya, gumbo, or stuffed bell peppers recipe. You can use other types of meat in these recipes too.
That yellow stuff at the head is just fat and adds flavor to recipes, so don't waste it. =)
P.S. Adding cayenne pepper to a recipe does NOT make it Cajun.