Is the prayer of the congregant valid if the Imam prostrated only once in the last rak'ah and was not alerted until after the Tasleem, then performed three prostrations of forgetfulness instead of two, and the congregant did not perform the third?

If someone forgets a prostration (sajdah) in the last unit (rak'ah) and remembers it a short time after the salutation (tasleem), they must go back and perform the forgotten prostration, then recite the tashahhud and give the salutation. Then, they should perform the prostrations of forgetfulness (sujood as-sahw) and give the salutation, and their prayer will be valid. However, if someone only performs the prostrations of forgetfulness without performing the forgotten prostration, their prayer is invalid and they must repeat it.