The Lounge is rated Safe For Work. If you're about to post something inappropriate for a shared office environment, then don't post it. No ads, no abuse, and no programming questions. Trolling, (political, climate, religious or whatever) will result in your account being removed.
Unless you actually want to get suckered into that project...I'd say run. Don't even offer any sort of advice, because if they're truly elephanted, they're going to see you as their savior, and next thing you know they'll be offering you what might appear at first like a big fat chunk of change but will turn out to be a living nightmare.
Of course I know nothing about the situation beyond what you've already written...but I'd be extra paranoid about this sort of thing. You've already raised the red flags yourself: Two developers have already taken a stab at it and quit...
I think the logical thing to do is to rebuild this, using Angular or React, from scratch. Probably Angular, but a NEW project that can willingly copy old code, but never accidentally inherits anything that's not been vetted
AngularJS is in maintenance, it should never have been used. they should never revert. There is no such thing as AngularJS code that can't be ported to Angular4 (although it's up to 7, why are they on 4?)
Have your friend walk away, sue the crap out of the software company to try and get funds back. Raddevus makes an excellent point - this project has been designed WRONG it is never going to come to fruition. Cut your losses and start again. The absolutely worst thing your friend can do is to invest another cent in anything but legal action against the software house.
There are many of us out there who work for small software houses or are single developers but your friend got a lousy one, next time he needs to do some due diligence on the quality of the software house.
Never underestimate the power of human stupidity -
I'm old. I know stuff - JSOP
Than any man could bear
Rescue me before I fall into despairrow...
"the debugger doesn't tell me anything because this code compiles just fine" - random QA comment
"Facebook is where you tell lies to your friends. Twitter is where you tell the truth to strangers." - chriselst
"I don't drink any more... then again, I don't drink any less." - Mike Mullikins uncle
Anything that is unrelated to elephants is irrelephant Anonymous - The problem with quotes on the internet is that you can never tell if they're genuine Winston Churchill, 1944 - Never argue with a fool. Onlookers may not be able to tell the difference. Mark Twain
The layout plans for the new office the firm is moving to have been released, I can see a Window! I like to be able to see the outside rather than the Day/Night indicators we have at the moment... it also appears we are close to Kitchen/Tea/Coffee facilities and the toilets are out of earshot (a plus!)
It seems it maybe good...