zinc affects both male and female fertility. he could be low in zinc, which is pretty typical in ms and can contribute to ms-related conditions such as optic neuritis, and low uric acid.
she could be low in zinc too, especially after having one child already (pregnancy is a big zinc drain for the mother).
optimal zinc level is 18.2 umol/L (that number is a 'healthy controls' number which i found duplicated in a collection of studies looking at average zinc in health and disease states) but the normal range varies widely around that, from up around 20, down to as low as even 8 or 9 umol/L..
this couple could both look into testing their zinc levels and see if there's any point working out a supplement plan. it seems, basically, to be the prime nutritional suspect.
nb: if there is caffeine and alcohol involved in their pre-pregnancy lifestyle, those are both zinc-drainers.