If a mage casts Beast Friend on a swarm of creatures, and then directs them to attack another swarm of the same type of creatures, what happens? Are the two swarms able to differentiate between those creatures in their swarm and 'the enemy', or will each swarm also end up attacking itself as well as the other swarm?
Swarms are assumed to have a level of animal intelligence, particularly given that you are talking about them being controlled by Beast Friend. Remember for example that most swarms are intelligent enough to be able to split into two smaller swarms. Also, in real life creatures that form a pack or social group are usually able to tell the difference between members of that group and intruders.
As this is the case, I would rule as GM that a swarm can differentiate between animals in their swarm and others, and so wouldn't end up attacking creatures that make up its own swarm.