Is it? It's not a complete black and white issue. MC got rich ( early in his 20's) and then turned into retiree, someone wasting away with no real life goals or ambitions.
For a girl like her, it was practically a slow death. And their relationship also turned stale/passionless.
And I doubt she immediately jumped into sack with someone to get his attention, this was likely her last resort, after everything else failed.
And common', you have to see the irony here of people calling out a cheating partner complete trash, I guess everyone here is 100% faithful on every porn game playthrough. Or irl.
Imo, reconcilliation with her should still be an option.
It's likely though player will face a final option to lose all his money here ( on Liam's treatment), probably leading to one of better endings.