Are there any restrictions to mix alcohol with other things? A friend of mine mixed whisky with milk and then he turned sick. he got rashes all over his face. But when i searched a little bit it turned out it's fine to mix whisky and milk. I want to know are there any limitations for mixing or can we just add anything we want to alcohols(mostly any) for better taste.
The main hazards of mixing alcohol with milk are that you may denature the milk proteins and get curds. Unpleasant, but probably not likely to provoke an allergic reaction.
Does your friend have any nut or pollen/tree allergies? Depending on what the whiskey was casked in, there could be some residual allergens that were the actual culprit.
Mixing Alcohol is fine, though it's best to use common sense when doing so and not to go too overboard doing so. It worth checking other possibilities as well contributing to the unwellness: Did your friend eat before drinking, what was eaten? As for the rashes, that sounds like some sort of allergic rection.
By personal experience, Mixed Alcohol effects people differently with me, I can't drink Jager-Bombs or mix two Different wines. But I have many friends that can.