As far as I know, you cannot directly exchange your BTC to DAI, and upon checking, Uniswap[1] has the most volume right now. However, you Uniswap only has DAI/ETH trading pair which means you need to trade your bitcoin to ethereum first, and then trade your ETH to DAI.
Yes he can as long he can convert his bitcoin to wrapped btc. Uniswap have liquidity for that. He can also convert that wbtc into dai, even tha pairing is via usdt or any stablecoin cause uniswap can convert it directly, just need to incurred some fees. Regarding conversion of btc to wbtc, not sure where this is done on dex but can easily be converted if you send it ti binance.
I forgot the exchange wallet but there is a mutlisupported wallet that no need kyc and can convert btc into dai directly. Will edit for the name.