I haven't seen the film. But looking over the shooting script, I note these points.
Mr. Greenleaf doesn't believe Marge. After she says she "just knows", he replies
Marge, there's female intuition, and
then there are facts.
Dickie had many dark secrets that he kept from Marge. MacCarron mentions an assault at
Princeton, a pregnant girl who committed suicide and possible involvement in the
disappearance of Freddie Miles.
MacCarron knows Ripley wasn't a student at Princeton, but does believe that he was there in another capacity:
Nor did they think to check whether a
Thomas Ripley had ever been a student at
Princeton University. I turned up a Tom
Ripley who'd been a piano tuner in the
MacCarron and Mr. Greenleaf believe that Ripley
was close enough to Dickie to know some of these secrets and are glad he hasn't revealed them to Marge. McCarron says
Mr Greenleaf appreciates your loyalty.
He really does. Marge, she's got a
hundred theories, but there are a few
things she doesn't know. We hope she
The money is partly because of the friendship they believe Ripley had with Dickie, and partly a bribe to keep him quiet about Dickie's past:
Mr Greenleaf also feels there was a
silent promise in Dickie's letter to you
which he intends to honor. He intends to
transfer a good part of Dickie's income
from his trust into your name. He doesn't
intend to give the Italian police any
information about Dickie's past. He's
rather hoping you'll feel the same.
So MacCarron knows that Ripley lied about Princeton, but doesn't realize that he is a complete fraud. I would guess that he supposes that Ripley is an actual friend of Dickie who wanted to make himself appear to have a higher social status than he actually does.
I hope this is an accurate reflection of the dialogue that actually appeared in the film. But even if it isn't, it explains how the issue was dealt with in a late version of the script.