Investing in a deal halfway through Rehab

Hi all, I have a friend who is a local experienced flipper and General Contractor. He bought an old Victorian in a rapidly gentrifying part of Sacramento. He has finished the remodel and is electing to BRRRR the property, a duplex. Because of the lot size there's an opportunity to add a 750 sq. foot ADU in back. He doesn't have the capital to build it but estimates he can get it done for $50K and it would increase the value by $75K -$100K. I could provide the funds, but I'm unsure of how I would go about setting up the deal on paper. I have a finance background, so I am certain that I'm just over complicating this. Does anyone have experience in anything like this? I'd appreciate feedback on any of these topics