Ya that is obviously an acceptable reaction if your group was there before.In Rhiannon since we have 3 or 4 major clans we have a FFA(Free-for-all) rule where in any 6* boss I your waiting there and another clan runs up and your waiting for a group then you can attack within a few seconds and it's FFA which is fair becuse is a tank from clan A Is waiting for dps and healers and a tank from clan B attacks and will pot until help comes then clan A's tank will attack and it'll be Ffa and then at that point it's who ever gets the most dps
However, the person who rushes the boss is likley a higher level than the group waiting, so it is usually futile for the group to attack because the rusher has good dps. The whole group doesn't need to just do more damage than the rusher, one person in the group needs to out DPS the rusher for the whole group to win the kill.
So if the rusher is a higher level and can dps, best bet is for the group to wait and see if he dies and the monster resets, then attack.