No, you can't tell your friend that you know why he's struggling for money. But maybe when she brings the subject up next time you could pose the 'hypothetical' suggestion that there's more to his financial problems than meets the eye, i.e. has she wondered if he's drinking/gambling/got another woman.
Risky though, as she could be offended at any such suggestion and you really don't want to destabilise a good friendship, especially as she probably needs her friends in light of her being recently widowed. On the other hand though, even if she appears offended, she might just give it some serious thought when she's alone. Which in turn could trigger a heart to heart with her son.
It is a difficult decision and I don't envy you.