This would generally be expressed in the plural. In fact, all numbers other than one will be plural, which the caveat that "one quarter of a pie" is singular, while "0.25 pies" is plural.
(Though "1 1/2 friends" is fairly nonsensical for the specific case. "1 or 2 friends" would be more likely, though the original answer could be considered cute, I suppose. "2 1/2 children" is a classic expression for the number of children in average families, but only when it's meant to be taken as an average. None of the families in town have 2.5 children, but they can have 2.5 children on average.)