Around the world, there are various contractors who provide electronic warfare training for military units, using specially-configured business jets. L-3 Communications is just one of several who offer this service:
http://www.defenseindustrydaily.com/lease-and-learn-l-3s-ew-training-aircraft-05378/
Not saying this was the case in Germany; the skies over Europe are extremely crowded, and this might have been an incident where the Lear stumbled into military airspace, or the Eurofighter got outside its MOA.
Limited flight space is one reason the Luftwaffe has spent millions on flight training in the U.S. They’ve had a presence at Holloman AFB, New Mexico for decades.
Updated info indicates the Lear jet was providing some sort of training for the Luftwaffe. Might have been towing a gunnery sleeve; during EW training, jets typically don’t get close enough for a mid-air.