How often does this happen?
You recruit someone new (or you are that new recruit yourself) and one of the significant #management challenges you need them to tackle is a historical "people" issue - usually a person or a clique - and you know it's going to be tough because:-
a) the predecessor wasn't unaware of the situation but they did not manage to resolve it;
b) it's historical - it's been going on for some time and the sorting of it needs to be swift, supportive and decisive AND leaving no room for claims of unfair expectations when past behaviours and performance had (by virtue of them not being tackled) been seen as acceptable
c) this person is new to the role (and perhaps the organisation) and that's a stressful enough place to be as it is
So, how can you support them?
You can tell them they have your support
You can ensure they have the skills and the confidence to deal with the situation you are about to deal them
You can encourage them to rehearse with you, how they intend address the situation
You can help remain focussed on the organisation's mission and values and how these can be used in the situation
You can tell them you know how much of a challenge it's going to be
but above all,
You can ASK them what support they need from you in confronting the challenge and GIVE it
And, you can me human and be kind