Welsh Rarebit is a savory cheese sauce (cheddar or Swiss with other savory components) served over toast.
In order to make a Chinese-style Rarebit, I would think you would have to save one part or the other (the toast or the cheeses sauce) in order to call it a rarebit. Making a smooth, savory Chinese sauce to pour over your Welsh Griddle Cake sounds good to me... as long as it's not sweet, maintaining the savory aspect.
Authentic Chinese or not, I've always liked Cashew Chicken Sauce/Egg Foo Yung Sauce (the same thing), and in my opinion, would serve as a good base for making the final sauce your own.
Your project sounds creative and is a very nice thing to do for your friend! I hope he appreciates it.
"Food is our common ground, a universal experience." - James Beard