How about:

De la part de quelqu’un qui pensait que … ?

In spoken language, “venant de la part de” could get abbreviated to “de la part de“.

Frank’s answer is good; other alternatives with possibly other shades of meaning:

Quand on pense que c’est toi qui … !

Et dire que c’est toi qui … !

As Ise proposed me to do it in the comments to your question, here is my proposition :

“Venant de quelqu’un qui pensait que… ?!”

Of course, regarding other answers, this is a very spoken form, but from what I understand of your question, I think this fits perfecly 🙂