Rev, Im sure youre capable of doing good work but dont you think it would be more appropriate to schedule work on a day when you're off work so you have the time to complete the job. Leaving someone there because you have to go elsewhere like your other job isnt much consolation for the person youre doing the work for.
Since you were committed to doing the work for someone that drove that far away, the appropriate thing in my opinion and this is what I would of done is to call in to work telling them you can't make it.
You really have to make a choice, its either you stick to your full-time job or you go into business fulltime fixing cars. You must allot the time for finishing the job, especially a customer that travelled a great distance to give you their business.
I don't think youre a bad guy but some planning is needed when you take on these projects.