Is the ghusl valid and must the prayers be repeated, or must the ghusl and prayers be repeated, in the event of forgetting to wash the feet after wudu during ghusl from janabah, then remembering that during Friday prayer?

The sufficient ghusl (ritual bath) is that which involves the washing of the entire outer body with water, accompanied by the intention (niyyah). Wudu (ablution) is not necessary for it. If you washed your feet during the ghusl and did not abandon the intention for ghusl, then your ghusl is valid, and so are your prayers. However, if you did not wash them, then your ghusl is incomplete, and you must repeat all the prayers you performed with that ghusl, including the Friday prayer (Jumu'ah) as Dhuhr. As for whether you should repeat the entire ghusl or just wash your feet, there is a difference of opinion among scholars on this matter. The precautionary approach is to repeat the entire ghusl and to repeat the prayers.