The idea is to minimize/reduce that risk and that can be done by having the right teams and people in place.

Maybe reduce your hours or pursue a different career path, in addition to the income provided by your rentals?

And since many lenders won't loan  on any purchase that is too low that reduces it even farther.

Sometimes wells have reduced flow in the winter due to water table fluctuations if its a shallow well, so you may only need to supplement with an occasional water delivery.

The downsides on the line of credit are the risk of the line being frozen by the bank or reduced entirely so when you really need the money it might not be available.

What opportunities are there to reduce expenses or push rents?

I have arrived at a rough estimate of 2010's cash flow by using 2010's rent and subtracting PM fees @ 10%, Vacancy (using 2018 rates with a floor of 4%), and Taxes+Insurance+CapEx reduced by 15% (to account for inflation).

If it's something like divorce or sudden, large, unexpected medical debt, that's one thing.

But I also don't forget about diversification because every investment is risky so to reduce these risks I created my property portfolio.

He could reduce the rent by the late fee amount and evict on non payment.If possible he should try to get any evidence that the tenant moved out, has a different address etc.