What's my exposure to any bad faith here?

[Geoffrey Pierce]

Out of state SFH property in arrears. A son is negotiating with his owner mom who is behind. The son needs a cash partner (me). He has another investor offer on the table for 350 cash, but the son wants to buy/hold and is offering me 50/50 equity partnership.

I was told no to the scenario below by a mentor, but didn't have time to get the why. He just half-joked "Get the deed, or don't do the deal."

The questions: 

1) In what ways can he screw me? 

2) How can I protect my position; contract. 2nd mtg, ...? 

3) Is the deal doable/worth it?

Thanks.

The deal: Purchase Brrrr via sub to with 26k down from me for arrears/rehab. Balance payable at ($xxx a month for xxx months) I become 50% equity partner. If I want an early exit , I get 50k at 18th month, otherwise... I'll negotiate some more pending this being worth pursuing.

The house:

435k arv

272k mtg

  26k arrears & rehab + 29k profit wanted by mom

327k purchase = 75%

2100 rent x .85 vacancy = 1785

1735 mtg service