I'm sorry you are having these issues.
I can certainly understand the conservative approach they are taking with this.
However, and these are just my, non-lawyerly observations...
While there is little to no case law acknowledging an option as a form of equitable title, there is case law acknowledging an option as a negotiable asset. I think if push came to shove a court would agree an investor has a right to buy an option and to sell that option without the assistance of a broker.
Where many get into trouble, and this is where I think your investor friend has problems, is most investors advertise the property, which they don't own and not the option to buy the property, which they do own.
FWIW, I don't like how options are used most of the time. I like clean transactions that stand up to scrutiny. But, I do believe an investor can buy an option on a property giving them the right to buy the property at a set price and later sell that option to someone else as long as the contracts do not prohibit it. Done properly, only the final buyer is at the closing table because the option exchange has already happened.