Which structure should I use

[dx4100]

I think you have answered your own question with the emboldened parts above. I would walk away. Especially as you have already said that although he would be responsible, you don't think he actually will.

Yup... Its complicated by the fact I owe him a favour

[tractor]

...

Yup... Its complicated by the fact I owe him a favour

I understand, you could just simply code the stuff for him and leave it at that, no ties, favour complete

I guess it depends on how much effort is involved. If it is too much, tell him you will design it then test it when someone else has completed the coding

[kingcook]

I will invoice him from my own company on a monthly basis.

As far as the business of splitting 50/50 goes I can see all your points. He is going to be the senior one. I don't want any involvement outside of developing the site for him. I have other things to concern myself with.

I need to think of a structure that allows him to make money and keep going but also recognise I did the work initially.

I owe the guy a favour. If its all just a lot of wasted effort then its not going to end my world

Cheers for all the feedback!

So by invoicing him every month, what are you invoicing for? What if he decides to stop paying invoices? You don't own any of the business IMO.

If you're serious about it, i'd consider setting up a separate limited company, split whatever way you feel appropriate. 50/50? 40/60? etc

I would keep your existing limited company well away from this.

Also, think about what you said, "I don't want any involvement outside of developing the site for him.". What about maintenance or other improvement if/when the customer base gets larger?