@Jim Petrosky what other qualifications does this partner have because your REI company is going to go nowhere fast if he gets his way. The power of leverage. If you need to explain to him why his plan is a terrible one, then I would reconsider his employment with your company.
If you paid cash for a $150k house and make $1500/month rent and assume $750/month in expenses, then your cash flow is $750/month, or $9,000/year. It would take you 17 years to buy another $150k property. What does a $150k property look like in 2040 compared to a $150k property today? A 12"x14" shed will cost $150k in 17 years with inflation the way it is.
Let's also take a look at your ROI for the same example. You are making $9,000/year with $150k buried in the walls of this house. That's a 6% return WITH all the hassle that comes with owning properties. Inflation is 7% right now. Could you make at least 6% investing in literally anything else? Absolutely. More cash in the deal, the lower your ROI. Less cash in, higher ROI. I always put in as little of my own money as possible so I can invest that money in more cash producing assets.
Someone above also mentioned you can buy 4 identical $150k houses, using leverage and get essentially the same cash flow. Not to mention the appreciation and loan paydown on 4 houses instead of one. If you're asking about scaling, your friend is on the extreme wrong end of the spectrum. Buying all cash on a LTR is not a good strategy.