The investor’s portfolio I am advising currently looks like this:

25 SFRs

Value: approximately 1 million

All properties owned, no debt

Monthly cash flow: approximately 10k

What are the best options for this particular investor to continue growing this portfolio?

Have any of these properties been rehabbed in recent memory? I would pick a handful of this investors properties with the newest roof, hvac, water heater and then have the investor refinance 40% of the home's current value to a bank lendor. 

I think a mortgage for 40% of the home's value is a good number because if the economy takes a hit the investor will still be OK. They will presumably also maintain a decent cash flow.

I would take his new money and either continue to buy new properties or hunt for rehab deals and then once rehabbed and rented out go back to the bank and refinance and repeat that process over and over again.

@Rob Bianco thanks for your response!

If I understand you correctly, you are advising a sort of modified brrrr strategy - modified since these houses are owned already but strategically pulling out a conservative amount of debt.

The houses are mostly C class but some border on B-.

Yep. We're on the same page. 

It's simply important for the investor to go through which of his properties are easiest to manage and where large repairs are not expected anytime soon. Take the cash from the bank and scale further.