Then, set up a policy requiring 2 people to sign for all purchases, and pay for things by company check only. Establish a formal expense reimbursement process. You can probably get an accountant or bookkeeper to help you set this up.
If this person was initially a friend and productive, and suddenly started behaving differently, there are three possibilities.
1) He was always a bad person, and you just didn't notice.
2) He has given up on the company for some reason related to the company, and is being passive-aggressive, plus inherently somewhat dishonest.
3) He is having personal financial/emotional/etc. problems.
Not sure how you would determine which of the above it is, or how best to respond.
I've had embezzlement happen at a company I owned, and it ended up killing the company (the scale of embezzlement was high, and it wasn't a normal tech company). Ultimately, the problem is a lack of process/controls, adequate enforcement and supervision, and hiring immoral people. As a result, I am much more cautious with all of those things now.
I think you're more doomed due to lack of founder vesting than anything else.