The comments by 5 Star were made in a letter sent to ALDE recently as part of attempts to forge an alliance between the two groupings.
The would-be link up never materialised and 5 Star has remained in the EFDD group, the openly Eurosceptic group which it shares with Nigel Farage's Ukip.
But, speaking to this website, Ukip's Roger Helmer launched a withering attack on 5 Star, calling its defence of the euro as "bizarre."
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Helmer, who defected to Ukip from the Tories, said, "This document reads like a desperate attempt to assert that the policies of 5 Star and of ALDE can be aligned, when in fact they cannot.
"It is my understanding that a key policy of 5 Star is to campaign for an Italian referendum on the euro, with a view to taking Italy out of the common currency.
"The idea that the euro has proved stable and resilient against external shocks is bizarre."
5 Star sent the letter to ALDE in the run up to the contest for Parliament's new President. In it, 5 Star, which is widely seen as being Eurosceptic, praises the EU as a "global champion."
Other senior political figures were quick to comment on the proposed 5 Star/ALDE link-up, including Glenis Willmott, Labour's Leader in Parliament, who said, "It's very hard to see any political affinity between these two parties. I can certainly understand why questions are being asked as to why Ukip and 5 Star have chosen to sit together in the European Parliament."
UK Tory MEP Charles Tannock said, "At a time when after six decades of US support for European integration, US President Donald Trump is on the record of supporting Brexit and asking which country will leave the EU next - and is advised by Nigel Farage who wants to implode the entire EU - I strongly welcome any comments by major party leaders for supporting the underlying tenets of the EU at a time of existential crisis."
Tannock added, "Grillo is an influential Italian opinion shaper who seems to be reining back from an EU phobic line held by some in his movement, which is wise and shows political maturity and judgment.
"I hope his group break with EFDD soon and join a more mainstream parliamentary group soon. Extreme versions of populism, nationalism, isolationism, and protectionism are not going to solve the problems of our continent or the planet."
Former ALDE group MEP Andrew Duff commented, "Grillo's letter proves that Guy Verhofstadt was right to think that he could prise away 5 Star from extreme anti-EU positions, or at the very least split up the 5 Star between nationalists and reformers."
He added, "I am sorry the tactic did not succeed this time - but the indications are that it will in time. The idea that no Italian can be a member of the Liberal group in the European Parliament is of course nonsense. Many Italians are natural Liberals."
The UK's former Europe Minister Denis Macshane said, "Beppe Grillo gives new meaning to the term political weathercock. One minute he is praising the EU and the euro to the skies and then suddenly he is back in bed with Nigel Farage. Beppe makes Berlusconi look like a statesman in comparison."