I don’t think it is advisable to hold their currency for them, it is better to open an account for them and give them access to the platform so they can be in full control and make them understand fully the operation and security, though you will still have access to the account pending the time your contract with them ends, I once did this with someone I introduced to BTC, I bought BTC for them when it was high which was in my personal wallet but by the time BTC fell, they became impatient and had to request for their full money back from me which I could not provide since I was not the one controlling the market, I tried explaining but it got worse till I was sued for it, first question I was asked at the law firm was if they had access to their BTC personally as at the time I bought it for them which I said NO and that is how I lost the case because it looks as if he borrowed the money which was time for me to pay back. If you still insist on holding though, I think exodus wallet is still more secured.