The question is whether it would be pursued under criminal law or civil. If under criminal law, you would expect the state to bring a prosecution if there was s

The question is whether it would be pursued under criminal law or civil. If under criminal law, you would expect the state to bring a prosecution if there was sufficient evidence and be responsible for all costs. However, in the UK for example, cases of breach of fiduciary duty are not often of interest to the police unless there is a public interest angle. Consequently, I presume that the OP is talking about a civil case where costs would be incurred to bring the case. Although there is likely to be an ability to claim some element of costs from the losing side, there is still obviously the risk that the case may not be successful regardless of how solid the case is due to the inherently risky nature of litigation.