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Hola Ελισάβετ 

this use of "le" in singular instead of the plural "les" with a plural referent is very common in Spanish. We use it so often that for me, as a native, the use of les here in this sentence would actually sound a bit odd. I would naturally use "le".

Here is what the Spanish Royal Academy RAE says about this. It is in Spanish, but you will probably get from the text that this use is very common. If you scroll down, the explanation is on point 6 : "discordancia en el uso de los clíticos"

Saludos

Inma

discordancia le-les