We did have about 1/5th of our angel money left when we decided to call it quits. There were a number of reasons for doing so at that time. Primarily, the remaining money would not have gotten us to where we wanted to be and we were not able to raise more money due to our inexperience (first time founders). We did return the remaining amount because that's the right thing to do. No need to burn bridges.

No - he'll probably have got a small proportion of his stake left.

If they were able to repay their seed capital, then it's a self sustaining business - there's a very thin line between success and failure here.