AFAIK, you need to have 20 xrp to activate that wallet. But that should suffice to activate that wallet. I've sent 20 xrp to my wallet as well years ago to activate it but I don't know in your case if you have to deposit another 20 xrp just to activate it because that's how it went for me.
It is 10 XRP and not 20 XRP. Please read what we have posted before posting.
First, did you send at the correct address? If it says inactive address then you might need to activate it first but since you already processed the transaction, then better to ask the support on if you can still get the money since most of the site says that if you send at the wrong address then it is already your fault and the site is not responsible to it.
That is another thing. Maybe he sent it to a wrong address. He should be able to track the transaction on the blockchain. But he said the withdrawal is not yet completed on Polonix. But that is kind of strange.