Almost impossible to answer your question in the form asked. If Sephiroth were different how?
However going on what is known. If you play (or have played) FF7 there is a scene in the inn after Cloud and party leave the capital city that gives background on how Cloud and Seph. know each other. The idea put across is that Sephiroth was cold and distant and not a friendly type from early. Then got even worse. So it is unlikely they would have been friends. At best they would have been battle comrades.
Actually, it is a high possibility that Cloud and Sephiroth would be friends if Sephiroth changed. Sephiroth was the son and experiment of professor Hojo, who after becoming fascinated with the entity Jenova, injected Jenova cells into Lucrecia (the bearer of his child.)Later in his life, Sephiroth joined Soldier and met Zack Fair. Through a series of missions for Soldier, Sephiroth eventually found out about Hojo's experiments and turned rogue. Zack Fair, who at this time, was friends with Cloud Strife went in search of what happened to Sephiroth; Cloud followed. Zack and Sephiroth fought. After Sephiroth was defeated, Experiments began to form clones of Sephiroth. Zack and Cloud were captured and experimented on, thus becoming "clones" of Sephiroth. Zack died in an escape attempt with Cloud. So if Sephiroth had changed, which is basically impossible, Zack would still be alive and Cloud and Zack would be working with Sephiroth. Sephiroth’s distant personality was there even before he found out about Hojo’s experiments so it would take a while for him to accept new friends. (Cloud got his buster sword from Zack and Zack got it from his mentor Angeal.