This is truly sad ...
Only time will tell if its acutal love between the older man & the woman ..
I, like Muileag, also questioned if it is merely a physical relationship. I truly dislike infidelity to my core - makes me feel like spitting - its nothing but destructiveness that effects everyone that, is of course, if their loved ones are aware of the truth. Most notably the parents of the female. Even in today's society, a woman is still considered to be a *ho** or promiscuous on many accounts including an affair.

I've seen infidelity highly defended w/great passion by some on SF as being an uncontrollable urge when meeting a twin flame which, of course, dispells the theory altogether of working together for betterment. Its like comparing themselves to the fictional Vulcans from Star Trek who are powerfully driven by Pon Far to mate or die. Urges can be controlled w/the use of reasoning, ethics, morals & respectability to guard against grievous errors in judgement. Unfortunately, the woman choose not to use any of them to decide upon the affair which was important enough to risk her marriage. Some people outweigh such decisions for the adrenalin rush produced from pushing boundaries to see what they can get away with. I don't think in her wildest dreams she ever expected to account for the affair, instead, may have thought she would be the one to leave the marriage on her terms/at her convenience for whatever reason. She also may not have known what she wanted or what her lover's thoughts were on a full time relationship much less marriage.
If it were true love from the get go as some claim then it should have been strong enough to do the right thing by withstanding time w/patience till the person became free from their commitment ...

Uncovering the affair forced them into accountability as well as make a decision about their relationship. They may not have been ready to be together, but felt bound to it in order to disprove it as a fling in order to restore credibility w/family & friends to make it easier to accept them as a couple as well as welcome them into their homes ...
One thing, I'm absolutely certain of is that it tests their trust in one another on a daily basis. A relationship stemming from an affair is filled w/far more complexities than many people realize, which of course, is not necessary information unless of course it helps to deter a person whose thinking comes from below the belt. Unfortunately, I've witnessed many things in my lifetime such as this - I can only wonder why this *hi* is shown to me...
Regardless of could be transpiring, they chose this path for better or worse. As always, I feel compassion for grown adults whose stupidy was driven by ego ...

In addition, the saving grace for your friend's son is he will not be forced into dealing w/some other predicament that could've stemmed from her ease in lying, covertness, deception & disrespect ...
IMO, the most important thing is that he is free from karma debt ..
The onus is on them ...
I know this is a great concern of yours, but know that his broken heart will heal in time ...
And I do hope he is making every effort to do whatever it takes to come to terms w/the betrayal & anger for his own well being as well as to enable him to cultivate a new relationship w/out fear based any reservations ...