I can understand her not wanting to believe that he was a paedo, it makes their whole life together a lie and a waste and no one wants to believe they have been that close to someone like that.
Marriages / partnerships are predicated on trust, which helps to sustain a stable, comfortable domestic arrangement, because however much we like to believe we 'know' the other person or understand them, we don't. So we rely on trust to fill the gaps in that knowledge. When that trust is breached by one or the other, then the injured party has the true nature of the significant other revealed to them - betrayal - the reaction to that can be quite catastrophic, essentially everything falls apart; past, present and also future and it casts that person back to the beginning of the evolution of the relationship.
Occasionally the betrayed partner will compartmentalise that betrayal, or mitigate it, as a defence mechanism for the psyche (or ego, if we're referencing Jung's theory of the ego being largely responsible for feelings of identity and continuity), and it's not a conscious decision, to the point where the denial is seemingly apparent to onlookers, but an incongruent concept to the them - they don't see it.