What Happens when a Masbooq Remembers his Missed Rakaahs only after Seeing another Masbooq?

Q: Salaah-10: two persons joined the salaah late and after the Imaam had completed, the one doubted how many Rakaahs he had missed, so he follows the salaah of the other. Is his salaah valid?

Answer: The salaah of the person in the above question is valid and he will not need to repeat it. His salaah will not be valid if he followed the other person as his Imaam. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2