I think it would depend on what the mono was looking for in the relationship. But picking up a new friend would be very low on the list. In my case not on the list. Its a nice goal and from all accounts possible but I think it is situation dependent. The hope is that they can get along and tolerate the situation and each other.
For me I saw no benefit.
I wonder how many close metamour relationship continue after the source relationship has ended. A non-poly example would be if my sister ever divorced her husband I don't think we would ever have much contact after that...unless accidental. I know I would not seek out any communication....I don't now.