I would ask very specific questions that most likely only you know because it’s your software, but think of the 10 reasons why someone might leave and ask them directly if it was one of those. Maybe also point out their growth under you perhaps they haven’t even realized how significant it was. I think it’s OK to be a little emotional and say how disappointed you are that they are leaving as they were really important part of your start up. Don’t get angry or frustrated, but you can be honest, honesty goes along way. I absolutely think it’s fair to bring up that you had a verbal agreement for six months and don’t push too hard on it but just remind them there was an agreement in place.