Frizzbonce and I think that the Relate counsellor saying this is completely the wrong approach and serves to make the clients feel worse, because it immediately gives the unfaithful party "permission" for being unfaithful and the history re-writers of damaged relationships, to find examples of marital dissatisfaction.
I read a brilliant paper last year about this and about how infidelity therapists need to change their approach. If clients come to Relate with a one-sided domestic violence issue, no counsellor would dream of telling the couple that the violence was a joint responsibility of the marriage and that it was merely a symptom of problems in the relationship.
No-one is responsible for someone else's behaviour choice - and therefore their infidelity.
Whatever problems existed in the marriage, infidelity is actually an extreme behaviour choice, when others existed.
A Relate counsellor uttering the words "It's never the case..." is also the wrong approach, because that instantly betrays their own belief system, when they should be permanently curious and not sticking with a dogmatic hypothesis.
The reality is that affairs do happen in marriages that were happy and many unfaithful parties will be honest and report that marital dissatisfaction was not the driver for their affair; there are a myriad of reasons why people have affairs and it is lazy and unhelpful to assume that the affair was a symptom of marital discord, when it is often actually a symptom of something else entirely - in the person who chose to be unfaithful.
Unfortunately, from this dangerous hypothesis, other flawed concepts follow - that a "happy marriage" (whatever that means 
) is an insurance policy against infidelity, when it evidently isn't. That a faithful partner can prevent infidelity if they behave a certain way in the future. That if a partner is adoring enough, has sex enough, pays attention enough, this will keep the horror of infidelity from their doors.
FWIW, if a Relate counsellor had ever said this to us when we were coping with an infidelity crisis, we would have spent the entire session pointing out the flaws in this hypothesis, because it is lazy and fails to understand the true complexities of infidelity.
There is a world of difference between understanding with hindsight, how a relationship became vulnerable to infidelity and working on it thereafter to reduce the risk, but doing this in a vaccuum and without examining the other factors (individual vulnerability, societal beliefs and lifestyle vulnerabilities) doesn't ever get to the nub of the issue and is why long after Relate, couples still haven't got to the heart of what really caused this and effected the behaviour changes that were necessary in the first place.