Do you think it is wise or unwise for a lady to help her partner with timing?
I think of that as more of a social issue than a technical one; It depends on the relationship between you and the person you're dancing with.
For example, female teachers will often subtly guide their male students onto the right beat. However, they tend to have the skill to pull it off under the radar. As far as the man knows, he was on the right beat all along. And even when it is recognized by the student, it will probably be considered acceptable by social standards because it's a teacher/student relationship.
If you're dancing with a random stranger at a social dance, you have to be a bit more careful about how you approach the situation. Again, with enough skill you can probably keep your back-leading under the radar, but if it becomes too obvious, it may not be met with enthusiasm. Some men may be very appreciative of the guidance, but the ones with big delicate egos will not. So it's as much a test of your people skills as it is a test of your dancing skills.
As for your partner, that's entirely a personal matter between the two of you. You are welcome to make up whatever "rules" suit your partnership. If the two of you can't come to an agreement (for example, if he'd rather dance off time than get your help), then you may just not be a match. If you want to stay with him, you'll probably have to live with the fact that you won't convince him otherwise. But you can still, to some degree, develop your subtle back-leading skills over time to provide undetected assitance.
For me personally, if I was following someone who was off time at a social dance, if I couldn't subtly guide them back, I'd just let it go. The goal in social dancing is to adapt, and sometimes that means adapting to someone's skewed (or non-existent) sense of rhythm. You'll always have a better time when you play on the same team. (This is a lesson I wish more leaders would learn, but that's a topic for another day).
On the other hand, if I had an amateur couple entering a competition, and the man couldn't hold a beat, then sure... I'd tell the lady to do what was necessary to get him on time. In that case, I believe the ends would justify the means. But that's because the goal of a competitor is very different than that of a social dancer.
Regards,
Jonathan