Co-inventor and Section 2870 of the California Labor Code

My partner and me developed a product, and the split is: I developed 97-98%, he developed the rest, the idea was conceived by both. He did his portion on his own time and equipment, and it uses general knowledge about computer networks, not related to his employer's products, and built using completely different technologies. However, his company builds other products related to networks. He has an agreement that he assigns his inventions to the company, unless they fall under the Section 2870 of the California Labor Code. He also has to notify the company in writing about his inventions, with a proof that they fall under this section.

1. What kind of proof should he give to them? He wrote a letter describing the product and stating that his portion was conceived and developed on his own time, equipment, not related to the company etc. What else?
2. We would like to ask for some kind of a letter from his company confirming that they don't hold any intellectual property in this product, or its part. I don't see a problem them agreeing, my partner contributed only about 30-40 hours worth of work, and his part is open source. The other part is copyrighted on my name. However, I don't want any trouble in the future, it mostly worries me that they can claim owning the idea. How should it be called and what should be included into the letter?
3. We can also offer them a license, but would it be a bad idea? They don't develop or anticipate products like that, but they can use it. Can a "customer" relationship create a "related to the business" connection?

You are asking detailed Qs about a specific situation. There is no way anyone can provide proper answers based solely on the information you have provided.

You need to retain a lawyer to examine all of the relevant documents and facts. Only then can you receive detailed, on-point answers.

Disclaimer: This information does not constitute legal advice and does not establish an attorney-client relationship.

Answering myself. I hope it's useful for somebody's else co-inventor situation.

The project is 2 autonomous parts, each developed by each person autonomously. So, he copyrights his part, and I copyright mine (only fair). He's disclosing his part as a nonassignable invention (and if they want, they can download the open source), and states that he's not obliged to disclose inventions by other people using it. As for the idea, we also split it - he owns his ideas for his part, I own mine. While I was developing my part, the ideas and decisions were mine, but I asked him general knowledge questions about networks and he provided me with answers. Answering questions is not an invention.