I know that the general guidance is that emergency funds aren't to be invested, but given our financial security and the fact that we already have a personal e-fund in a safe, non-volatile location, would it still be crazy/unwise to invest this 15k in an S&P500 index fund in a brokerage? 

Especially if they profit from the transaction. 

Took my chance the first time in 2017 and never looked back.My transaction was always one property into multiple.

While I did not work with other agents or lenders for this particular deal, my professional experience ensured a smooth transaction and effective project management.

While I’ve been handling smaller real estate transactions for about a year and a half, I knew I needed guidance to break into this space.

Do your due diligence.furthermore, are there any burnout houses close by, is it in a safe neighborhood, is this in a rural area, is it a metropolitan area, what's the economy like there, just because other places nearby are renting doesn't mean that your house one block over is not next to something that is uninhabitable, or poses a security risk, or maybe near an abandoned field, a graveyard, an airport, many different factors there to consider.  

And nobody is telling a lie in the transaction. 

I thought I was safe dealing with him because he's all about family too.

So in my opinion, it teaches me how to use short-term leverage to build an income snowball...then once I get an efficient and ever-increasing machine...I pivot that into getting more long-term assets (using safe, long-term leverage). 

Funny, I was hoping to equip myself with some ideas so I could propose some transaction options to them right off the bat--but I agree with you that that is probably not the right move.