Lock out

Yesterday I was called out to a property where the the tenant had left a key on the inside of the door which prevents the key on the outside working. I have been called to this property before with the same problem. The issue with this door is the letter box is down at the bottom of the door which makes gaining entry a problem but with the right tools and a boroscope (camera) the customer was in his property with no damage and no lock change. I advised the customer he would be better having a thumbturn cylinder fitted this will stop this happening as the knob on the inside stops you leaving the key on the inside