travel and rhum

I've gone on hikes all my life, though I've never done the really long ones. However, it has always helped me relax and get back in touch with myself, especially when I hiked through forests. From what I've read, apart from the usual "physical activity = better hormonal balance and more happy hormones", plants and trees release some kind of chemicals that physically improve our mood.

The question I have is – how effective is this? Could hiking serve as a good weapon in dealing with mental issues, especially the ones that are chronic? I'm asking because I'd really like to try and help a friend of mine who gained a bit of weight and got a little depressed lately. He's open to traditional therapy, but the issue is that he can't afford it right now, so something that could be done for free would be a great way for him to kick off that self-help journey.