Tom Sandoval's Late Friend Ali Rafiq Found Collapsed in Parents' House

Tom Sandoval and Ariana Madix's late friend, Ali Rafiq, collapsed in his parents' house before he was eventually found dead ... TMZ has learned.

Ali's sister, Rabia Rafiq, tells us her mother found Ali on the floor of their St. Louis home Friday afternoon, and she says it appears he had fallen and hit his head on a table.

We're told Ali was found dead on the floor around 4 PM Friday ... about 3 hours after they believe he passed. He had called his father from the house about 4 hours earlier.

Ali's sister says her brother had no known illnesses, other than asthma ... and the family didn't believe that was ever a serious issue for him. We're told there were no signs of foul play, and no indicators as to why he collapsed.

Ali was a big foodie who loved to post pictures of his meals, but his sister says he was not overweight or unhealthy.

The Rafiqs are Muslim and Ali's sister says their religion calls for bodies to be buried ASAP ... so the family buried Ali on Saturday, right after his body was released to the family.

Because Ali died Friday and was buried the next day, we're told Tom did not have a chance to fly out to St. Louis in time for the burial. We're told Tom got to his hometown Sunday and has been spending time with Ali's parents, and visiting the gravesite to say goodbye to his friend.

Ali's sister says Tom's parents attended Saturday's funeral and FaceTimed with Tom so he could feel like he was there ... and Ariana sent flowers.

As we reported ... Ariana looked somber Saturday when she got back into town from her trip to NYC, paying tribute to Ali with a social media post. Tom also shared a remembrance post.

Ali's sister says his death certificate cites "organ failure" as the cause of death, but the family is waiting for toxicology reports to learn what really happened.