Make the client feel welcome. At this point, they may still be open to having conversations but remind them that you are available to provide as much support as possible.
If they are willing to describe how they are feeling upon contact, then simply assure them that you are there to listen. Many people are unaware of the reasons for their mood changes at this point, so providing a listening ear is more than enough.
Find out what their interests are. Reassure them that you will work at their pace and that there is no pressure involved. Allow them to tell you what their expectations may be.
At these early stages, lethargy and self-loathing are the most present symptoms. Remind them that it is about the small wins. Help them to set smaller goals that are achievable to help improve self worth.