Annually. Choosing not to vaccinate is irresponsible and asking for trouble even if the horse goes nowhere. There is always the chance of tetanus, and flu has a way of finding its way onto the quietest of yards.
every year to the date. Should always be done. On the horses passport should be all the up to date vaccination stamps. I hope your friend doesn't take the horse anywhere, competitions etc, as their vaccs records should be 100% up to date.
People who do not vaccinate for tet at least are completely irresponsible IMO. Tetnaus can easily be prevent but lethal and heart breaking if contracted.
I can not understand the reasoning behind not vaccinating.
My vet vaccinates my horses at the intervals he suggests. This, for tetanus, is currently every other year. It would be extremely foolish to not vaccinate for tetanus. Flu depends on what you use your horse for and where you go.
Flu every year, tet every other year and myself for tet every 10 years, even if the doctors try to fob me off!! (Which reminds me to book my appointment)
My own horse has flu every year and tet every other year.
I tend to advise clients that tetanus is essential, as it can be contracted through a wound that you may not even notice. By the time the horse is showing symptoms it is too late. Treatment is very expensive and generally unsuccessful.
Flu on the other hand depends on where the horse is kept and what it does. Any horse kept on a livery yard or riding school should be vaccinated, as should any horses that go out and about to fun rides, lessons at other yards, hunting, competing etc. Horses kept on private premises in a stable population that do not mix with strange horses do not essentially need to be vaccinated against flu. (I work in a very rural area and many ponies will never leave the farm they were born on).
But remember with any animal, not just horses, the cost of the vaccine is always much less than the cost of treating the disease!