2 types of lime. NHL, naturally hydraulic lime, which has a chemical set, and Pure Lime (non-hydraulic lime) which sets by carbonation, a very slow process. If you were to reuse the latter it would need to be burnt at high temperature. The former not reuseable, technically.

What your friend may be referring to is when you are initially using Pure Lime mortar, and it has not had time to set, it can be reused, knocked up again. If you use pure lime it will never set if you keep it away from the air. So in a bucket covered in water. It will stay like that forever as long as the water does not evaporate. NHL will set under water. You are right in saying some old tech isn't bad. Some are great. Just ask the Romans....Off to work I go. Have a great day.