Don't say anything to your son in law
I had no idea my ex was cheating, and while it would have been hard to hear it from anyone I wish someone had told me. If he is cheating then she is at risk of picking up STIs from random women, marital money is being siphoned off to pay for dates with another woman. There is the risk of an extramarital pregnancy
I think OP would know if the marriage was entered I to polyamorously. And if she doesn't know and consent it can't be polyamory
I do agree that you need to tread carefully OP, and don't mention to your son in law at all.If he is cheating she might be blissfully unaware that he has not got a sudden interest in fishing etc. She may be glad of a bit of time to herself
A big problem if the news comes out of the blue is that she will be wrong footed and may not think clearly, ideally she would need to keep that she knows under her hat, and get legal advice and collect all the information she will need before he starts hiding assets etc
Can I suggest a great website called chump lady, she has written a book called leave a cheater gain a life. But the website gives you a list of red flags for cheating, what not to do if you suspect, what to do instead, worth taking a look.
What seems to be very common is that most times it is not the first time, just the first time they got caught
And most cheaters start showing the same behaviours, almost like there is a handbook they all study.
Worth looking at so maybe you could have a chat if she is feeling down at being neglected