In Zindel's The Pigman, why does Lorraine become angry with John about going to Mr. Pignati's house?

In Paul Zindel's The Pigman, Lorraine makes a prank phone call to Mr. Pignati one night, which ends with John convincing the senior citizen to donate ten dollars to a fake cause. John promises Mr. Pignati that they will come over and collect the money soon. However, Lorraine has different feelings:

"The next day Lorraine chickened out and said she wouldn't go with me to collect the money. 
'Give me one good reason,' I demanded.
'Because it's wrong to take money from an old man, that's why'" (31-32).

Lorraine clearly does not feel comfortable taking an old man's money, especially after hearing how sweet Mr. Pignati sounds over the phone. John is accustomed to taking advantage of other people because he is the puller of many different pranks over the phone and at school. Lorraine only makes the prank call to Mr. Pignati during a prank calling game with John's friends. Lorraine is nervous because she has a conscience and cares about other people. As a result, Lorraine tries to back out of meeting Mr. Pignati. The following excerpt shows Lorraine becoming angry with John about collecting the ten dollars from Mr. Pignati.

"'I've been thinking, and I've decided we'd better go over and collect the ten bucks.'
'I've been thinking, and I've decided we'd deginitely better not,' she snapped.
'We're not doing anything bad,' I insisted.
'Ha!'" (36).

Eventually, Lorraine goes to visit Mr. Pignati because John tells her that old people like visits. He even manipulates her by saying that the old man might be suicidal if they don't go visit him. Lorraine does go to see Mr. Pignati, and they end up becoming good friends.