That is pretty much the opposite siting a bigleaf hydrangea prefers. Morning sun and then shade from hot afternoon sun is ideal. You can make this situation work but you will need to water frequently to avoid excessive wilting in the heat and the plant may not thrive as it would do in better sun/shade conditions.

My suggestion would be to transplant these to a more suitable location and replant with a more sun and heat tolerant variety like Hydrangea paniculata (rather than the mopheads or Hydrangea macrophylla).

I'm rather surprised that the nursery sited them there. If they chose mopheads for that spot, then shame on them. If you chose them and they didn't explain to you why it wouldn't be a good choice, shame on them too.

It also appears like the mulch is dry but I cannot tell if it is moist in underneath. Make sure that the soil is maintained evenly moist at all times for all of those hydrangeas (the transplanted mopheads in their new location and the suggested paniculatas in the location shown in the picture. You can experiment with 1 gallon of water per watering per plant in Spring and more in the summer months.

As long as the plants look fine in the morning I wouldn't be worried about losing them, but with continued heat stress and sun scorching they certainly won't thrive.