Is it permissible for my husband—as a bus driver who cannot perform the prayer at its appointed time—to combine prayers by performing them earlier (jama’ taqdim), and if so, when and how is this done?

If the distance the driver travels is approximately 85 kilometers, it is permissible for him to combine and shorten prayers. He may combine Dhuhr with Asr and Maghrib with Isha, and shorten the four-rak'ah prayers to two rak'ahs. However, Fajr prayer is not combined with any other prayer. If the distance is less than that, he is not permitted to combine or shorten prayers unless failing to combine would result in harm to life or property.