There's no direct one-size-fits-all solution because mental illness is complex and very personal. Anecdotally, depression can be more of a symptom than the underlying root cause in a lot of cases, especially if trauma is involved. A good therapist and support from peers is invaluable for identifying deeper causes and patterns. On a day-to-day level, depressed people may need gentle encouragement from someone in their proximity, something to break their routine in a positive way, support if they're frustrated with what little they're able to accomplish, and help breaking down big tasks into small pieces that they can more easily summon the energy for.

Medication can certainly help, as well as exercise and diet - but if someone's not there yet, simply pointing to those and treating someone like they're just not putting the effort in is extremely damaging. It reinforces the catastrophization that can occur and makes people feel like they'll never be able to take control of their lives and it's their fault. And if deeper causes are involved, they may not be able to explore those around someone telling them to just exercise more.