A mini-split is an air conditioner where the compressor is outside, and the fan and evaporator coil is inside, connected by two hoses for the refrigerant. These can be purchased as heat pumps strictly for heating purposes, or heating&a/c. It depends on how cold it gets there whether it would work as a reliable heating device on the coldest days. It would work fine for most of the year, probably...it all depends on how cold it gets on the typical coldest day.
Not all pellet stoves are created equal...some have the ability to light themselves, hold a large quantity of fuel, and should work reliably for probably at least 24-hours...it would be an issue if you were going away for longer, so a backup plan would be required.
There are a bunch of wall-hung, through-the-wall furnaces that can be run off LP.
There are also some nice water heaters that are designed to also provide space heating...he'd have to put in some baseboard heaters or, better, find some refurbed cast iron radiators or low profile heaters. Those would provide a more even, comfortable heat that won't dry the place out like a forced-air system would, and when arranged properly, would provide a more even heat.
Before you consider anything, though, you'd probably want to try to figure out the actual sized unit required...this is called a manual-j calculation. There are some free on-line calculators that aren't too bad, but a good analysis would require a pro with the right tools. The inputs required are the size, location and type of windows, the size and orientation of the cottage, the amount and type of insulation in the walls, floor, ceiling, and an idea of how air tight it is. Some work on making the place air tight would go a long ways towards minimizing the energy costs and improve the comfort radically for often a small investment.