What version of LR do you have? LR 6/CC has a brush associated with the gradient filter where you can erase certain select areas. In LR 5 or before, try playing with the shadow and highlights sliders to reduce the effect on the darker wall or person. If that doesn't work, use the adjustment brush instead.
Thank you Joe! I appreciate your help.
You are welcome. Out of curiosity, and to help others, which option was the correct one for you?
I was using Lightroom 5.7. From talking to you, I've took a subscription for CC.
I still haven't learnt how to perfectly expose the person, yet darken the sky (Gradient filter), but I guess that will come with time.