Is it explained how Achilles and Astolfo can lend Noble Phantasm? I have only seen the anime so I am not sure if this is covered in the novels but for Charon to ask for a Noble Phantasm would indicate he knew that Achilles could lend Noble Phantasm to anyone or he specifically knew Achilles would run into Astolfo. From a previous question I asked it was indicated that this was a unique circumstance of Achilles being known for lending his Noble Phantasms and Astolfo borrowing Noble Phantasm. Is this a unique interaction between Achilles and Astolfo or is this just a power of Achilles or Astolfo?
In actual legends, Achilles is known for giving away relics (e.g. For Patroclus to impersonate him, lending him his chariot and armor), and Astolfo is known for receiving them in his legend (which is why he has so many different Noble Phantasms, which he collected through his adventures). These circumstances allow the two to transfer and recieve other Noble Phantasm, which would not normally be possible (although there are exceptions, such as Berserker from Fate/Zero).
The anime itself is a poor adaptation, and glosses over or outright changes a lot of context and background information. Therefore a lot of thing the audience should know are left out arbitrarily.