Unless your situation is exceptionally unusual, you don't have a "visa for France". You have a visa for all the Schengen states which happens to be issued by France.
When you applied for this visa, you documented a certain trip you have planned. You're supposed to do that trip more or less how you told in the application you would, or you risk your visa being revoked as having been issued on false pretenses. But "more or less" does cover reasonable variations in how you arrange to get yourself from point A to B, as long as points A and B are what you claimed they would be. A stopover in Frankfurt is completely fair game, and whether part of your travel within the area is by bus or by air does not matter either.
In summary: A short-stay visa issued by France is valid for entering the Schengen Area in Germany and continuing towards your main destination in France by ground transport.
The friend who tells you that you must enter the Schengen area through the country that issued your visa is simply wrong.