When I have two guests I always make up two beds without an additional charge.
Do you know if your friend told the host you were expecting separate sleeping accommodations? If not and you can't find the extra charge mentioned in their listing or rates, if this happened to me, I'd ask the host to point on where the extra bedding charge appears in their property description or rates.
I charge a cleaning fee for 5 or more guests. My sister rented a cabin from AirBnb and she was expected to bring her own bedding. Depending on the hosts' circumstances, there might be an additional charge that seems out of the norm.
However, any additional charges should be explained on the property listing or quote before you make a payment, not after.