If it is nearly dead, it will not come back from the root system. They are annual plants and do not have a perennial storage system on which to draw energy reserves. Your friend's best option is to purchase and plant some new seeds. It's not very difficult to keep a marigold alive.

Here's the story on petunias. They ARE short lived perennials. They can be cut back or killed back and will often revive from their perennial root system. However, they will be exactly the same cultivar or hybrid. It is only if you collect and plant the SEEDS from that plant that the progeny will be of an unknown quality.

Personally, I wouldn't give up hope yet. I had the same dry-shrivel thing happen to a patch of mine last year. They were near-dead when I decided to move them to a better location.
Although both locations were full sun, the new location offered better water retention, and I mulched them, too. They recovered after a couple weeks and went on to be one of my better performers last year!
This would be um...Janie, or Lemon Gem...not sure which...it was a petite yellow thing.