Camellias like their soil to be kept as evenly moist as you can so, make sure that it has 2-4" or organic mulch at all times. While moisture issues could contribute to some yellowing leaves, in June, camellia japonicas (like your mother's plant) like to drop old foliage as Daisyl pointed out.
One other thing that I would check... is the plant in part sun? japonica leaves cannot stand afternoon or evening sunlight during the Summer months. Full sun is ok during winter or early Spring though.
Since I could not see what could possibly provide shade after 11am or so in the picture, just check that something will give it shade. Sasanqua leaves can withstand full sun but japonica and reticulata leaves do not.
If the soil is a little sandy, consider watering more water and adding a 1/2 to 1" layer of organic compost every Spring.