Yes I agree with Anke - wethers would be best. I suppose I'm a bit of a softie, but if there is the chance that an animal destined for slaughter can have a reprieve, then I would rather go that route. From your point of view, if you are not making a loss by selling the sheep for mowing, then why not. Just check that your friend knows how to care for them properly and not to overfeed with 'treats'.