You and "your buddy" bought bitcoin together and both of you two know the password too?
Can I take some legal action against my partner?
What valid proofs you have if the password was changed by yourfriend? I know, it's to strange while the wallet password has changed and your friend also disappeared (too much coincidence).
Why did you have to start with 1 BTC though?
Because
as that gives you enough capital to never make a loosing trade with they system of trading I have.
Since both of them owned it, probably the its both must sit and talk what happened. Air your concerns to your "buddy". It can be resoleved by both of you, it needs only time to do it. Talk honestly. I think you have personal issues with your "buddy", hope you resolve it offline.