In The Pigman, how do John and Lorraine become friends with the Pigman?

John and
Lorraine become friends with the Pigman - real name Mr Pignati - in a rather unusual way, by
playing a trick on him.

Bored and dissatisfied with their lives, John and
Lorraine, along with two other friends take to making prank phone calls to random people picked
from the phone book. It is Lorraine who turns up Mr Pignati's number, and calls him pretending
to be from a charity and asking for a donation. He sounds so pleasant and friendly on the phone
that Lorraine feels ashamed of gulling him, but John takes the game a step further by arranging
to call at his house to collect his donation.

When John and Lorraine go to
his house, the friendly impression he made over the phone is fully reinforced.

When Angelo Pignati came to the door, I wish you could have seen
him. He was in his late 50s and was pretty big, and he had a bit of a beer stomach. But the part
that slaughtered me was this great big smile on his face. He looked so glad to see us I thought
his eyes were going to twinkle out of his head. (chapter 5)

John here admits to being quite overwhelmed by the sheer affability of the man. He is
such a cheerful, obliging person that John and Lorraine simply can't help taking to him, and
they all become good friends. In fact they start visiting him regularly. What started out as a
mischievous prank on their part unexpectedly leads to an unlikely friendship between a lonely
middle-aged man and two disaffected teenagers.

 However, this friendship also
leads to . John and Lorraine unintentionally become friends with Mr Pignati; they also most
inadvertently help to precipitate his death. He gives them the run of his house, but on one
occasion they have a party there where the guests get out of hand and end up trashing the place.
This causes Mr Pignati to have a heart attack, and shortly afterwards he succumbs to another.
John and Lorraine are left feeling very guilty over their role €“ however unintentional -  in
his death. So their friendship with Mr Pignati ends up having enormous consequences for all
three of them, which none could have predicted.