The Core objective of Mental First Aid (My perspective only!)

The objective is movement.

Get the concerned person moving in a non-negative direction, no matter how slowly, as soon as possible.

If someone is unhappy about something, even asking about it will almost always help.
If someone is depressed, getting them moving physically, even just a walk, is enough to change mood significantly.
If someone overweight is eating unhealthily, telling them to change even just one small thing, like telling them to eat one leaf of vegetable, or ordering less carbs will let them know it is at least possible.
If someone has a bad dream, asking them about it alone will help.

This is the bottom line of helping anyone psychologically. And of course, there is a lot more which can be done, many better techniques that lead to faster results.