Answered by Shaykh Faraz Rabbani
Question : Is the Friday Prayer Considered valid if one joins at the very end, before the salam?
Answer : One is considered to have caught the Friday prayer if one joins the congregation before the imam’s first closing salam.
However, delaying getting to the Friday sermon (khutba) without an overwhelming excuse is sinful, because listening to the sermon is necessary (wajib).
[Shurunbulali, Imdad al-Fattah; Ibn Abidin, Radd al-Muhtar]
And Allah alone gives success.