[hresvelgr]

They're probably just taking a break. time_sunk != output.

I "waste" time at work quite often but that's because I know when I am in certain moods of productivity. Forcing myself to work when I'm not in the mental state to do so is like beating a dead horse. Perhaps your co-founder also recognises this.

Have an honest discussion. Keep your concerns free from judgemental language; for example: "When I see you playing video games it makes me concerned about what your priorities are and I want to have a chat about it." If they're reasonable, they'll be straight with you. If they ascribe any judgement in your words (assuming you use non-judgemental language) then they're probably feeling attacked, in which case, I would de-escalate. The key thing here is empathy. If you handle the situation with decorum, and they do not, then you likely have more problems beyond your co-founder wasting time.

edit: coworker to co-founder