Hard to comment without knowing exactly what words were used. For example, you can say how disappointed you are in a person without calling them a fking ** so whilst the former is objectively true, the latter may be something worth apologising for without recanting your very fair and very justifiable disappointment. Although whether it's fair and justified also depends on whether your friend's account of events is entirely accurate and not conveniently obscuring any wrongdoing of his own.

Be careful of people giving you dead certain advice when they only have a loose appreciation of events. Same goes for giving advice yourself. Let's say hypothetically (and this is purely a hypothetical, I'm not inferring anything here) he had cheated on her but left that part out. A detail like that would drastically change what stance you take but for someone choosing to omit that information, they would take your advice/support under false pretences, as validation they have done nothing wrong.