Is it permissible for my husband to give a training course to three people, one of whom is an unveiled girl not adhering to Islamic guidelines? And is the money earned from this course permissible (halal) or forbidden (haram)?

The permissibility of earning money from training depends on the subject of the training. Whatever is permissible to train on, it is permissible to take payment for. As for the ruling on the work itself, it is related to observing the limits set by Allah, self-monitoring, offering sincere advice, enjoining good, and forbidding evil.

If the trainer is confident that they will not be tempted and adheres to the Islamic regulations when dealing with a student who is immodestly dressed, such as lowering the gaze, offering advice, enjoining good, and forbidding evil, then there is no sin upon them. Nevertheless, avoiding training immodestly dressed women is better and safer for one's faith and further from temptation.