Do keep your expectations low. Just extending the invitation is not a guarantee that it will be taken – or even that it should be. Remember that you are not responsible for that person’s happiness; only for inviting them to share in your own.
Do be a sympathetic ear, within your own very human limitations. Although it can sometimes feel that a depressed person has the same conversation, or types of conversation, over again, having someone listen from time to time does have a positive effect.