(Hypothetical)

If I am traveling for business, I may take a flight to e.g. Frankfurt just for a 2 hour meeting, and then immediately return. I would like to know what EU regulations stipulate if the outbound flight is delayed so much that I could not make the return.

I have seen one other question related to what the airline probably will do, but I would like a statement about EU regulations.

For instance, is the airline required to refund the whole ticket price? Presumably, if I see that I cannot make the meeting, it doesn't make sense for me to travel at all. So I would simply not travel. Or is the airline perhaps only obligated to make sure that I get to the destination and back? For instance, in this case, perhaps the airline just has to get me a new ticket which says that I fly to Frankfurt, and immediately return on the same plane?

I understand that if my original flight is delayed due to non-weather related events, and I miss my connection, then the airline is generally responsible to get me to my destination. Furthermore, I understand that the EU has regulations laying out what should happen if a delay is longer than X minutes. But it is unclear to me what should happen if the delay interferes with the return flight (not just the connection).