I've been taking LDN for around 6 months, and I've been taking a chemo drug, Cyclophosphamide, for about one month (I had an intense burst of 5 treatments in 8 days, now it will be once every couple of months).
The thing that needs to be remembered is that, at least a few months ago, LDN had not undergone any clinical trials for MS, so the notion that LDN should not be combined with this drug or that drug is based on theory, not on what actually happens in the body, which could easily be more complex.
From my experience, though, taking LDN, chemo, other supplements/diet modifictions, there does not seem to be any interaction.
As for how long the drug stays in the system, my hair started falling out last week, almost three weeks after my last infusion of chemo. It might be the case that there is no more Novantrone in the person you care for's body, but it probably continues to do its work for a couple of weeks after the drug is gone.