The ingredients remind me of the Bruce Edwards Curry House Cookery recipe.
That's the only other time I can recall a lemon going in!!
The yoghurt is intriguing too. And, as Chris says, the 10 chillies (for 2kg of onions! I can't imagine that would make a typical korma...or CTM...or any other mild curry?

). Not to mention the spuds, of course......though that's nothing particularly new....but 3 spuds (albeit "small") to 2kg of onions would seem to be a relatively high proportion of spuds (assuming he uses all of them and blends them into the gravy?)

We'll soon see, I suppose....

JB, have you ensured that he's happy that you post the recipes here? Maybe he'd like to partake in the forum? You can ask him those questions, too, perhaps?