"the parents covered the remaining amount with funds borrowed from the Heloc on their primary residence."  

And that could be anywhere from $58.32 to $2 million.  Always more fun to answer questions here, when guessing is allowed.

But is the amount of the investment relevant, anyway?  What is their agreement?  Partnership agreements don't have to be in writing, but it's always a good idea.  Just because they're related doesn't mean they are allowed to avoid good business practices.