PNP clarification please

[Kergan]

Here is the situation:

A group of 6 people are leveling in SolA:
25 warrior
25 rogue
28 enchanter
27 monk
29 wizard
30 cleric

Things are going well until an SK zones in and attacks the cleric. The SK is only in range to the cleric, and kills him. Upon zoning back in, the SK tells the cleric to LNS. Fearing punishment, the cleric leaves the zone and the exp group disbands as they cannot continue without a healer.

So my question is, does the cleric have to LNS, or does the fact the group is still there allow the cleric to stay? If allowed to stay, can the SK continue to attack or would that be considered griefing?