eazybee
'Interfere' is a poor choice of word.
I think the mother has watched the son's first marriage disintegrate, and sees similar behaviour from him the second time round,which she fears will cause the relationship to founder.
She has tried subtle advice which the son has disregarded so there is nothing more she can do , other than tell him a few blunt home truths if the relationship does fail.
Perhaps she doesn't want him back home again.
The mother's fears are her business, not her sons's.
She should ditch the "subtle advice" and accept that her son is her son and he is how he is.
If she doesn't want him back home she can tell him that the consequences of his choices are his problem (if it comes to that and they are problematic), not hers.
In short, she should treat him like an adult.