This card is fine. It's not bad. It's not great.
As has been noted below, it gives many allies an extra use. This is definitely helpful. Of course, then there's also the more obvious utility: 1 extra point of horror and/or damage that you don't have to take.
The cost is 1 resource and 1 card, no action.
In some ways, Dodge is better. It costs 1 resources and 1 card, no action. Dodge can let you avoid more damage. Odds are you're more likely to only take advantage of either the 1 extra sanity or health with Trusted, but not both. Dodge let's you avoid a whole attack which can be much more damage.
On the other hand, Dodge only works on enemies, not treacheries. Trusted lets you store damage away from your investigator. In addition, it can make scenario allies more hearty if the scenario tries to kill them (think Adam Lynch.)
Second Wind has a different cost than Trusted: +1 action but -1 card (because you draw to replace it.) It can heal more but it has a very limited play window for best effect. This card is best suited to investigators that take damage regardless of the allies they have. Mark Harrigan and Sophie, I'm looking at you.
This card is well costed and serves a useful niche. It's not bad. It's not game breaking. I'd recommend Trusted in decks that have 6 or more allies. Maybe 4 or more if you include Calling in Favors or if your allies run very useful abilities with a self-damage component, like Aquinnah or Beat Cop.