Is eating milk and fish together toxic and responsible for white patchy skin

No it is only a rumor. White patchy skin has not been reported after eating milk or cream based fish preparations. So it is safe to drink milk, buttermilk or eating milk-based dessert after eating fish.

Fish and milk together is certainly NOT toxic! Fish is often cooked in milk or served with a milk-based sauce and there is absolutely no reason why one should not drink milk after eating fish unless, of course, one has an allergy to fish or milk (or both). It is possible that your mother's cousin was mildly allergic to both milk and fish and that mixing the two exacerbated the allergic reaction (though such reactions don't usually make the skin white and patchy. They are more likely to cause swelling in the mucous membranes).

I am not sure if it's toxic but I too have heard that taking both together may be toxic.

That does not sound true to me.

I have never heard this and would find it hard to believe.

I don't believe this is true

I have no idea. It is probably an old wives tale. I had milk and fish together as a child and was fine. How else could people enjoy fish in parsley sauce? The sauce is made from milk.

We don't use either, so not sure what is causing this for you cousin. Maybe it is due to allergies. What did the fish eat? What did the cow eat?