alternator belt FROM HELL !
What trailbeast is saying is start with the obvious/simple and go from there. The deflection is something like 1in? What others are saying is you need to have the exact same size and shape.
What TrailBeast is trying
not to say, is that the OP didn't ask how much we know about belts, rubber, pully's and all the tech involved.
He said a friend changed his alternator and now the belt looks loose and it squeals at low RPM's.
He didn't ask what every possibility on the friken planet could be, or who knows the most about it.
New people need simple straight forward answers, not pages of discussion on it.
Now to the point, he said "changed the alt"
Not the belt, or the pulley's, or anything else.
AND he said the belt looks loose.
It aint rocket science, and it doesn't have to be made into rocket science.
Especially for a new person to mechanics.
He just needs to know that it squeals at lower RPM's because the belt is loose and slipping when the alternator is trying to replace the power used out of the battery after starting.
Tighten the belt until it can be pushed down about an inch midway between pulley's
If that does not do it,then a new belt may be in order.