But it also doesn't make sense, I don't know if it's a person who doesn't know much about bitcoin and is mixing things up. If bitcoin is yours, it makes sense not to want to mix it up. You can have a bitcoin that you bought in an exchange with kyc and another bitcoin bought in another way without kyc (that because it was without kyc does not mean that you are not going to declare it later). But it is not good that someone who observes the blockchain can know all the bitcoin you have, so it is better to keep it separate.

But as for the donations, as you say, he should declare them all. The fact that some come from exchanges with kyc and others from other sites without kyc is not relevant.