The devil is in the details.
As Tranquility said, the loss itself would not indicate a breach of fiduciary duty. It is whether the overall portfolio was appropriate under the circumstances.
Or, to paraphrase one of my favorite financial advisors, "Don't confuse strategy with outcomes." You can't control the outcome. You can control the strategy and ensure that the strategy is appropriate. Sometimes an appropriate strategy goes up in flames. Sometimes a totally inappropriate strategy comes up smelling like a rose.
The agent under the POA might be held accountable for the strategy, not necessarily the outcome. Just as a thought experiment, would anybody be upset if the agent's strategy had produced a positive 40% return over the same period that it lost 40%? I doubt it even though the strategy may have been just as inappropriate.
That said, the best suggestion is for the parent, or an interested party if the parent is not legally competent, to get as many details as possible about how the finances were handled and consult with an attorney familiar with the fiduciary duty of an agent in Virginia.