Owing Money

I had signed up for an activity that required a purchase of some equipment in order to participate. I planned on buying the cheap equipment. A friend of mine later decided to sign up for the activity as well. I happily spotted the fact that if we pooled money, the two of us together could buy better equipment but pay the exact same price we would have if we had been buying the cheap equipment on our own. My friend agreed. We bought the more expensive equipment. My friend made a small planning error that meant they were dropped from the activity. My friend expects me to pay them back for the equipment we bought together. I can neither return nor sell the equipment because we opened it. Do I pay them back? I feel like a good agreement would be to pay them half of what they lost, because it was still partially their fault. But my friend expects me to pay back the full amount they lost, and has confronted me about it.