Is it permissible for someone who missed Fajr prayer to pray behind someone performing Dhuhr prayer, and what are the implications in this case?

The first [option] for one who missed the Fajr prayer is not to pray it behind someone praying Dhuhr, in order to avoid the disagreement of the majority. The more correct view is the permissibility of praying a fard (obligatory) prayer behind another fard prayer, whether the other fard prayer has more or fewer rak'ahs. So, if one prays Fajr behind Dhuhr, he has the choice between separating from the Imam and concluding with two rak'ahs, or waiting for him until he concludes with him, and waiting for him is better.