My friend is about to build a new stable building for three horses, and an 80m^2 office building adjoining

He has asked me to help design the office.

Well insulated and air tight I said and he agreed!

Could the horse manure power a biogas heating system of 1 or 2kw?

and if so how?

I was suggesting wood pellets as woodchip is not available locally (all going to local power station!)

I've kept horses tony, and as you too are probably aware the droppings are generally compact and reasonably easy to collect, indeed your friend will most likely be collecting them frequently as part of good stable management. Dried manure is frequently used in the third world as fuel and I see no reason it couldnt be used here. I guess the key to success is the drying. I've seen one simple device like a wooden brick mould, You chuck in the droppings, compress them with your boot or a tamper, then eject, stack, and leave to dry, you could build up a summer store to last the winter. For the stove bit, if it were me I'd be tempted to have a small, remote boiler stove, a bit of hot water too, but there are some quite small room heaters that would I guess be OK.