Objective mediator. First, one should speak to a doctor or counselor. It helps to have a neutral person, whom everybody trusts and respects, mediate the situation. Ultimately, everyone involved must agree on a single goal that is in the best interests of the person receiving the intervention, not just imposing someone else's will on that person. Offer that person the chance to choose between reasonable alternatives.
Specifically. Please be more specific as intervention has many meanings. Sorry.