@OP what's your client a client of? Do they pay you in crypto or fiat? What service do you provide?
I'm wondering if it's worth clarifying with changelly that you're not a payment provider or providing a means of payment to someone else - it's likely banned in their terms to have a client that just interacts through you for making exchanges. I don't think in AML policy it's strictly possible to compel a third person unrelated to the initiation of a transaction to supply their own documents unless you make it a standard procedure or add it as a condition of their next contract.
Why would he need to explain to anyone who his client is? The client can be a large global company or just an ordinary user. What if the client agrees to do KYC, so Changelly requires KYC from the fourth party, the one who sent the coins to the OP's client? Where does it end? Who authorised Changelly to investigate?
Also, stay away from these sites: Changenow, Changelly, Simpleswap, Fixedfloat, and Morphtoken.
Whitebit should be added here.
btw. I never heard about Morphtoken.