What do you mean by loaning for the day?, do you mean hiring him out for a full days riding, or litterally alowing another person to care for him for the 2 days
I should think the fairest way of doing it would be to work out exactly what he costs you per month and then divide to find the daily cost and double it.
So if monthly costs are £300 divide by 30 x 2 = £20 a week for the 2 days
What would the loaner/sharer be able to do on their days as this may affect how much people are prepared to pay ?