Option that is frequently used to protect fallen allys is to stand in the same square and take attacks from enemies, or at least provide cover if someone is trying to finish off the fallen allies.
This would check if the square of a Prone character is shared with a Standing Character that is an ally, and provide Standard Cover.
Never seen such rule. You can take cover behind a creature larger than you, but there is nothing that gives auto standard cover when being prone behind/under a creature.
Option that is frequently used to protect fallen allys is to stand in the same square and take attacks from enemies, or at least provide cover if someone is trying to finish off the fallen allies.
This would check if the square of a Prone character is shared with a Standing Character that is an ally, and provide Standard Cover.