Freddie taking over at the stables would be an excellent move for him long term and one which I have been saying (not on here) was an obvious solution to a number of problems for a while.
BUT he has no qualifications and little experience, Joanne can keep running things but expecting a complete handover to happen quickly is unrealistic. It would be an ideal plan for Shula's retirement though, especially as she may want to cut back on the physical aspects.
It doesn't solve the issue of how long it would take Shula to train or the sacrifices she would be expected to make.
Incidentally I can't think of any Archer who would be keen to help at the stables other than Freddie.
Not many Archers are horsey, in fact in the whole of Ambridge there is very little interest in horses which is slightly surprising given the demographic.
Shula and Christine obviously are serious about horses and have qualifications/experience.
Chris Carter works with horses as a farrier but that doesn't mean he knows much about their day to day management and I don't believe he rides.
Justin, Lillian and Oliver all ride regularly but use Shula's livery services.
Alice has/has a pony, spearmint, although he has been forgotten recently. She last rode I think with Debbie when she was visiting. Debbie said something about missing riding.
Helen rode a couple of times during the Rob years. She referred to not having ridden since she was a child but quite enjoyed it. She hasn't ridden since.
Joe Grundy has two ponies, Bartleby and Gem, he drives them and keeps them on a budget at Grundy Field. Eddie and Ed help him with them so presumably have some knowledge.
Many farmers view horses as an expensive nuisance that use good pasture and earn no money. So I can't really imagine anyone other than Freddie taking over the running of the stables as they are.