Though it may cash flow well based on their current debt service, when you compare the income it produces to the equity they have is it actually even a good investment for them from an ROI perspective or would they be better off liquidating and redeploying?

so, you're maxing out debt service which makes it tougher to cash flow.with that said, no matter the purchase strategy, you should always make sure you're using actual numbers, and not random assumptions.for example - some people will throw out '10% for capex.' 

Owning a duplex at 20 is a huge accomplishment, and it’s inspiring to see you’re already planning your next move.From my perspective, real estate investing is a long-term game focused on building equity, paying down debt, and maintaining stability over chasing short-term cash flow.

The sentiment feels a lot like 2008, home prices are unaffordable, Price to earning ratio don't make sense, rents are high (which is a goo thing only of the homes weren't so expensive), I feel like it's a ticking time bomb with the debt crisis.

I am currently unmarried, no kids and recently out of residency with six figures in medical school debt.

Debt service is way up, revenue is generally down, down payments are way up, financing terms and costs are much worse.The time of fast scaling has passed, unless you're very creative. 

A HELOC is a short term debt instrument.

I agree with @Shawn Ackerman I would start applying the cash flow to debt pay down.!

How they think a national park is near you is laughable though lolTechnically Tigertail is a park but it would be a far stretch from calling it a national park.  

In my experience, these are also referred to as "QM" and "Non-QM" loans ("QM" = Qualifying Mortgage)And as consumer mortgages, these loans are generally issued under the person's name with the underwriting based primarily on the borrower's DTI (Debt to Income) ratio.