the first thing I ask myself in this sort of situation is whether the person is actually correct. part of being effective in a specific role is refusing to take

the first thing I ask myself in this sort of situation is whether the person is actually correct. part of being effective in a specific role is refusing to take on random work people throw at you. estimates can't be valid if key players are spending lots of time on stuff that wasn't on the schedule. if the person is really just being obstructive, explain to their manager (or have your manager explain to that manager) why you think that team should fix the issue. keep going up the chain until someone with authority knows who ought to fix the issue. it's possible you get all the way to the CEO without finding someone responsible for fixing the issue, but in this case the problem is with the organization, not the IC.