In the episode Damage, Archer boards and steals a warp coil from a Illyrian ship. The issue is, it will take the Illyrians 3 years to get home.
After stopping the Xindi, why didn't the Enterprise track down and help the Illyrian ship?
This is addressed in the EU novel Rise of the Federation: Uncertain Logic. In brief, Archer made some efforts to find them (by asking a Vulcan ship to scan for them in the general vicinity), but neither hide nor hair of them was ever seen again.
Reed lowered his gaze, embarrassed at what he’d almost brought up. But if anything, Archer was the one who still blamed himself for his act of desperation in his darkest days in the Delphic Expanse, when he’d stolen a warp coil from an Illyrian science vessel and stranded them three years from their homeworld at sublight to ensure that Enterprise would reach a vital rendezvous. He’d seen no other choice, and he had intended to return the coil if possible—but circumstances had forced Archer to leave the Expanse in haste to defend Earth from the final attack of the Xindi hardliners.
Later, once Archer and Starfleet had persuaded the Vulcan High Command to send a follow-up expedition to make diplomatic contact with the Xindi, Archer had requested that the Vulcan crew attempt to locate the Illyrian ship and provide aid and recompense if possible. But no sign of the vessel had ever been found. Archer could only hope that they had managed to contact a friendly warp-capable vessel and find their way home, with the worst damage being to Starfleet’s reputation. But he would probably never know for sure.