How much is an acceptable percentage for a ROI in 6 months on private money?  

But remember on assets that low every $200 is a percentage point on your return and if it slides by a few months from your forecast your returns again drop.

They usually will require less of a down payment and usually loan a higher percentage and allow you to loan 100% of the price plus the repair/rehab costs. 

ROI,  means return on investment and does not consider down payment, equity percentage, or if funds used for investment were borrowed, stolen or gifted or traded for equity.

Did everyone contribute capital in the beginning and this was his percentage of contribution?

They pay me a percentage, but they save a much bigger percentage by keeping their rents at market rate, getting tenants that pay and don't trash the units, etc.If I were in your shoes, I would cash out the equity on the home and use that for investing.

If you adjust the numbers to more normalized percentages this property won't look as rough. 

I would also recommend being a passive investor. leading your capital out to seasoned investor who are actively doing this process on a regular basis who know what to do and you just make a percentage on your money but also learning in the process. 

The minimum rent shall be increased by the annual percentage increase in the United States Bureau of Labor Statistics, Consumer Price Index for Urban Wage Earners and Clerical Workers, Midwest Region All Items, as of October 31 of each year times the prior years annual minimum rent.

I would definitely try to get the highest percentage of your financing as a HELOC that you can, just for the flexibility it offers.