It was a mistake to trust this service. Any service that shares private keys as a mechanism to transfer bitcoin can't be trusted. He should do what he can now to secure his coins and the way to do that is to sweep them to an address whose private keys only he knows.
OP clearly said that there isn't even a history of transactions, so how is this a trust issue from ChipMixer's part?
That's probably related to the different wallet files issue that i talked about.
Also, what's the difference between trusting to send 0.5 BTC to a service and trusting to send and temporary hold a (shared) private-key with 0.5 BTC? In both cases, you must trust them enough to send your Bitcoins and expect them to send it back.
We all use exchanges to buy and sell bitcoin. We have to trust exchanges temporarily while they have our funds. Sometimes even that temporary risk bites us in the backsides when the exchange runs away with our money. Here you are asking the user to trust the service permanently i.e. even after you've "withdrawn" the funds.
The other thing is they are doing things the wrong way by sharing private keys. What else are they doing incorrectly? It is a basic mistake they are making and puts doubt on their entire service.