As far as I know, and I'm not a doctor or anything and not giving medical advice, Mg supplements of any kind must be avoided if you have kidney disease. The reason being that the kidneys filter out excess magnesium from the blood, which can otherwise be deadly.
However, in your husband's case, I'm not sure how this applies. If his kidneys are still able to filter out the excess Mg, just that he subsequently has trouble urinating, I think he'd be fine from that standpoint. If his kidneys really don't work as they should, be very careful with magnesium.