They are very good with food restrictions and alternatives are available.
Contact MSC before you travel: this is really important and means they can assist a lot better. They will then ask your father to speak to the Food and Beverage manager: they usually provide an open appointment hours on a card in the cabin. They can ensure that adjustments are made in the MDR for food by checking the menu for the following day and choosing food in advance.
We found there were options in the buffet for Oat, Soya, and Almond or Coconut milk - not all were always available, but staff were able to get them if they had ran out. In the MDR is was only Soya I my experience, most other venues about the ship didn’t have any alternative milks when we asked (that’s not to say they might not usually, but we stopped asking as I’m ok with black coffee).