What is a valid US visa for an Indian cofounder in an accelarated tech startup

A friend of mine in India is a cofounder of a company in the US that has just raised 100k$ in seed money from an accelerator similar to YCombinator(very well respected), but does not have an H1-B. The other two co founders have US work permits. So the other co founders will be responsible for all the money related stuff , since my friend cannot be paid until he has a work permit. Their plan is to build the product and have an investor round on demo day in February . He has to be in SFO to start product development from Sept 15 without getting paid.

1. What are the options he has with respect to visas? India is not in the US waiver program. Does he go on a B1 visa or a B2 visa and for how long will it be given and what proofs would he need. Obviously he will not be paid there.
2. Considering that they will be enough to raise money in february, will they be able to sponsor his H1-B , since he co owns the company. IS there a clause that prevents this or is this allowed? If it isnt is there another visa category for this.

He has tried googling but hasnt found satisfactory answers in this particular case.

Does he go on a B1 visa or a B2 visa and for how long will it be
given and what proofs would he need. Obviously he will not be paid
there.

No, he cannot use B1/B2 visa, since working is prohibited in that status. The fact that he's not being paid is irrelevant.

Considering that they will be enough to raise money in february, will they be able to sponsor his H1-B, since he co owns the company. IS there a clause that prevents this or is this allowed?

This is allowed, but likely to be denied if he's already in the country and working.

One option would be to create a subsidiary in India and employ him through it there. After a year+, you can attempt L1, if H1B falls through.

Talk to an immigration attorney for a legal advice and guidance, I'm not an attorney.

I find the answer that he cannot visit on B1 to be mis-informed
A B1 is the type of business visa most overseas citizens can obtain easily. From Wikipedia’s B1 entry the activities you are allowed to do on a B1 visa are:

Negotiate and sign contracts
Purchase supplies or materials
Hold business meetings or attend/exhibit at a convention
Settle an estate
Sit different types of exams and tests held inside the United States

Writing code is no different from "preparing a contract" and/or "holding a business meeting" since both involve thinking and writing logic typically on an electronic device.

He/She could also be provided room/board as long as its not monetary compensation but simply an act of friendship/complimentary gesture. Seems like B1 visa is just fine as long as he is not getting paid any money.