Hi Sophie, he will be ok on his own as cats don't need company. If the lady is prepared to take more ferals on, I would bite her hand off at the offer as suitable homes are so few and far between.
One thing, the lady will need to try and keep him contained in one barn or stable for a couple of weeks so he gets used to being fed there and knows to come back (similar to when you move home with your cats)
Finally, he is neutered isn't he?
P.s. I am the lady that rang round all the farms and stables in my local yellow pages. I'm on Bedfordshire which is just up the road from northamptonshire.
Well done for helping this guy too