Lets say Franco decides to send troops to assist the French during the Algerian War, particularly during a time when Morocco is still under the joint rule of France and Spain. What would the effects be, if anything, besides Franco's popularity at home declining and Moroccan nationalists disliking him even more?
Franco was still pretty radioactive at this time - the last fascist standing. A French government which accepted military assistance from Spain, especially troops, would probably fall within weeks; would almost certainly be clobbered at the next election. It would also taint French rule in Algeria with fascist associations.
Franco's offer would be firmly declined.
Oh, yes, and Algeria was considered part of Metropolitan France. So here is a foreign government offering to "assist" France in governing its own territory.
What if Canada had offered to send troops and police to help put down the Detroit riots in the 1960s? What if France offered to help Britain secure Northern Ireland? Etc., etc.