When someone’s not at their best, help them, notice the best in them, encourage them.
To do this you can ask yourself: What is one of the qualities I appreciate in this person? Then ask yourself what small success you have noticed this person achieve today, it may be as small as they smiled or dressed in a beautiful colour, or said something kind to someone.
Notice these successes and qualities, however small. Keep them in your mind every time you think of them or interact with them. Remind them of their worth, by doing this you will slowly begin to draw the best out of them and help them feel good again.