[mmxiii]

This is a nonsensical comment. With the information we are given, there's hardly any basis to conclude anything about:

1. Who is betraying who

2. Who should take ownership of the startup in case of a breakup

The OP is asking for any relevant experience and advice, not for someone to irrationally moralize.

[crimsonalucard]

This is a nonsensical response ignoring the intrinsic relationship between morality and the information given.

Since the situation is intrinsically related to morality. My advice is therefore also related to morality. Just because the OP doesn't specifically ask for a moral answer doesn't preclude me addressing morality.

Betrayal and ownership are subjective in this context. However, there is still a majority consensus that can be arrived at based on the information given. Many will come to a single conclusion on who is the betrayer and who morally deserves ownership. When a person thoughtlessly steals his best friends girlfriend, what do you think the majority consensus will be?

[mmxiii]

There are a few things getting conflated here.

1. What the OP should do.

2. What the majority consensus is

3. How you subjectively feel about the majority consensus.

4. How the OP subjectively feels about the majority consensus.

In this case, there was not sufficient information given to really understand what the majority consensus is. Additionally, just because something is the consensus, doesn't mean it is necessarily the best course of action individually. Facing condemnation is a consequence, not physical law. The initial response took no time to give an appropriate course of action, and drew on your subjective heated feelings attached to your interpretation of the consensus.