Anyone have a good egg free flourless chocolate cake recipe?

You could try subbing 1/4c silken tofu per egg. I've done it in chocolate chip cookies and cheesecake with excellent results.

This doesn't answer all parts of your question: I haven't made it, so I don't know how good it is. But maybe you could try this Food52 recipe for Chocolate Mochi Snack Cake (https://food52.com/recipes/34383-chocolate-mochi-snack-cake) with a few substitutions. Someone in the comments successfully subbed the evaporated milk for coconut milk, and you could sub the eggs with applesauce or a flaxseed slurry. Since you want something denser than the Moosewood cake, eliminating the eggs might work to your advantage here.

Report back on what you did!

If he's open to chocolate dessert other than cake, there are wonderful mousses that use chocolate, avocado & sweetener. Can be garnished with any fruit he likes, or gluten-free crunchy topping.

This is a good, already vegan chocolate cake. If you have a gluten-free flour mix you like, I think this cake would probably take well to it:
https://moosewoodcooks.com/2013/11/six-minute-vegan-chocolate-cake/
It's based on a Depression-era recipe and there are lots of other versions, often called something like "wacky cake" or "crazy cake." There are a few examples made with gluten free flour kicking around the internet, and it seems like they were done with a straight 1:1 replacement for wheat flour.