Well, in the end it isn't the intention that counts, but the result. In the Natalee Holloway case he at least got Joran to speak about the evening. In Mabels case he was able to show that Mabel had lied to the prime minister and to the Queen and hence she and her husband were removed from succession. Mabels image never recovered from the report of Peter R de Vries and the press avalanche afterwards IMO, it probably never will. In the public mind she is a lying over-ambitious woman who shared beds to work her way to the top. It probably isn't a right evaluation of Mabels character but my guess is that that is the image that most Dutchmen have of her.
The show of Peter R de Vries provided some classic quotes and the Chilenian bandit (Charlie da Silva, former bodyguard of Klaas Bruinsma) on a horse, looking into the camera and saying 'Mabel, do you still remember me? I do remember you' is a classic that usually can liven up any birthdayparty. Other quotes like 'she was the woman of the tall one' (the tall one aka the vicar aka Klaas Bruinsa, a Dutch mafioso) and more are still well known too and there was even a song about the boat 'Neeltje Jacoba' where Mabel and Klaas Bruinsma were said to have spend evenings together.
The only way she could ever change this would be some sort of press campaign and a few tears in an interview might help, but that will never happen (thankfully). There is no reason for such a campaign and maybe the couple is even happier in the shadow than they would have been when they had to perform public duties (Friso doesn't seem very socially gifted).
Note that Princess Mabel still denies any romantic or sexual involvement with Klaas Bruinsma, she said she misinformed the Prime Minister and the Queen about how well she knew him (apparently it was more than the 'loose sailing contacts' she said in the beginning). According to her it was all hyped by the press and many lies were told, which is true according to a recently published study on the matter.
About Peter R de Vries: he does research after crimes, but his style can be too sensational as we saw in the recent reports (why was it needed to show a crying Beth Twitty for example). Apart from that I find him arrogant, cold, rude and too full of himself. Still, he has been able to solve some murders and other things in the past, dispite all this.