If the BTC's are still there, move them asap to a much secure wallet and probably you will be the only one who knows. Crypto is money and the personal interest of other people with other's fund is there.
I don't know on how long you've been together but with the explanation you have that made me think that your friend(business partner) took it. You don't trust your wallet with any person even it's your friend, I would only trust a bit my wallet to my wife or kids and parents.
Also the thing on my mind too and highly suspect that his friend is the only one who do took those coins and he's too merciful not to get those BTC's or sweep all the balance for it not to be soo obvious but eventually if a hacker or someone do able to access the keys then those coins will be sweep out totally.This is the hard part where you do trust up some people
in regards with money where anytime they would be tempted to stole it out specially if we do talk about big sums on here.We cant really be sure their loyalty anytime.