It is a hard one.
On one hand the mother may really appreciate you saying something...she may be feeling lost or like a bad mummy and may relieved to find out there is a reason for his 'behaviour' and it is not her fault.
On the other she may be offended and tell you take a long walk off a short plank
I think I would be the former but we are all different.
It sounds like the slow approach you are doing now may be the way to go...just talk to her in the morning about your DD's traits that he has also and maybe she will put two and two together.