A friend of mine says that a big part of the reason scientific and philosophical terms were put into Greek and Latin is because during the Renaissance only the educated wealthy learned Greek and Latin, and they were trying to keep knowledge and power out of the hands of commoners. Any truth?

This is a weird concept.

The real reason was that latin was the language all educated persons shared across europe, and science and philosophy have always been based on the exchange of ideas.

By writing in latin people in england, france, germany and italy were able to communicate with each other.

It is not much different from today, when scientific papers from all over the world are written in english more often than not.