Does misaligned serpentine belt cause it to slip off?

I have a 2007 ford fusion 2.3L with 90,xxx miles. About a year and a half ago I had the belt changed because I had a leaky axle that was slinging greass all over it causing it to squeak and it's was the original belt so I guess it was time for a new one anyway. Everything was fine up until about 6 months ago, I had a idler pulley that was in bad shape causing excessive noise replaced by a friend that works on cars and we just reused the same belt since it's fairly new. All was fine until a month or so later I began to get another squeak and I had no idea what is, not bad just on start up but after awhile of driving it goes away, I read online I may just be a misaligned belt so Ignored it for the past 4 months and this morning I get into my car start it and a hear a wword noise immediately after I turned the key and my battery light came on and lose of power steering, pop the hood and belt is off all the pulleys. Strange how traveling over 70+MPhil daily going 100 miles away for months nothing happens and simple start up takes the belt off. Any ideas on if I should get a new belt? Or does tightening pulleys make a difference?

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