This is a tricky one! Finding plants to mask the smell will be rather difficult, but some of those with powerful scents may help. You could try lavender, nicotiana, sweet rocket, sweet peas, stocks, jasmine, witch hazel, viburnum, verbena, choisya, roses, for example. Perhaps it would be better to consider planting a mixed hedge (also good for wildlife) to hide the sheds and underplant with a mixture of scented plants which come into bloom at various times of the year? If you get on with the neighbour, some of that smelly chicken waste would make extremely good fertilizer and quickly lose its' odour when dug-in the soil (just trying to think positively!)