How about: "We split the tasks among ourselves keeping in mind our strengths, but unfortunately the co-founder failed to deliver on his share of the tasks, acc

How about: "We split the tasks among ourselves keeping in mind our strengths, but unfortunately the co-founder failed to deliver on his share of the tasks, according to him he was affected by x,y,z. (His reasons) Then we failed to move from there and ended up shutting down the business." Let the employer judge whether it's his incompetence or real problems. Why pass a judgement needlessly? Deliver facts.