"It is not meet" basically means "it is not appropriate" or even "it is not right" (probably a better interpretation in this context). He's saying, more or less, that they shouldn't just leave Arthur in the state he's in, but someone should stay with him.
Meet, as defined in Lexico:
> [archaic]
> Suitable; fit; proper.
It doesn't pop up as a definition if you just search for "meet meaning", mind! You have to be sure to search for "meet meaning adjective", or some combination thereof.