sounds to me like "leaf drop" all though in my experience if it happens all leaves not just the new ones can drop. Tell them to be sure the soil temperature stays above 55 degrees so that the roots stay active enough to help cool the leaves. The sun is quite powerful through glass and can really heat up the leaf surface. There is some debate on what the minimum temperature is to keep the roots at, but that is what I have researched and works for me.

mike

It definitely has not affected old leaves like the leaf drop you menioned- it's just like the tiny unrolling leaf buds are dropping.

It sounds like it may possibly be a response to a lack of water. If the soil goes dry when news shoots are starting, I have seen my citrus trees shed these growing shoots. If the plant is in a dry environment (for example, if it is in a centrally heated house, especially forced hot air), the effect of any lack of soil moisture will be magnified.