In a recording of her comments circulated by the French embassy on Wednesday, Grillo blamed Lebanon's "political class" for the downfall.
"But what is frightening, Mr Prime Minister, is that today this brutal collapse ... is the deliberate result of mismanagement and inaction for years," she said.
"It is not the result of an external siege. It is the result of your own responsibilities, all of you, for years, of the political class. This is the reality."
Diab has been serving in a caretaker capacity since resigning in the wake of the catastrophic Aug. 4 Beirut port explosion. Since then, fractious sectarian politicians have been unable to agree on a new government.
Donors have long demanded reforms to stamp out state corruption and waste - widely seen as root causes of the crisis.
Diab had noted repeated calls for assistance to be linked to reform, but said "the siege imposed" on Lebanon was not affecting the corrupt - an apparent reference to politicians.
Lebanese were running out of patience and "linking Lebanon's assistance to the formation of a new government has become a threat to the lives of the Lebanese and to the Lebanese entity", he said, saying the country was a few days away from a social explosion.
Grillo noted that France, with the backing of other powers, had organised two conferences on Lebanon since the Aug. 4 explosion and was working on a third.
"France and many partner countries around this table have not waited for this call to help Lebanon," said Grillo, adding that last year Paris extended $100 million in direct assistance to the Lebanese people.
Though Diab's government is serving in a caretaker capacity, she said it could take steps including implementing a social safety net with World Bank financing.
Diab’s speech echoed accusations made by Hezbollah leader Sayyed Hassan Nasrallah on Monday, in which he blamed the United States for the suffering of the Lebanese, accusing it of forcing countries not to provide aid for Lebanon.
(Reporting by Imad Creidi; Writing by Tom Perry; Editing by Samia Nakhoul, William Maclean) ((email@example.com; Reuters Messaging: firstname.lastname@example.org))