What's overlooked about this card is the fact that the cost may be paid by any player at the table that has access to resources, it doesn't have to be the player controlling Ioreth. This makes her a very valuable multiplayer ally - if you absolutely need to spend your resources elsewhere this round, your buddy can trigger healing for you. I love this card.
I’ve been playing a lot of multiplayer recently and Derndingle Warrior is great. Self Preservation and Ent Draught are a great combo on him. Unexpected Courage is also useful allow more chances to defend, if you have access to . Even with Beregond on the table he gets a lot of use! I often run my Ents with Hobbit heroes and low threat so having Sentinel means your fellow players can concentrate on attacks on enemies who’ve engaged them while you defend. Great card.
This seems like one of the generically best boons to choose from this scenario. 1 cost for 3 attack, plus a little bit of progress too.
Though I wonder where the flavor texts on the new boons in this scenario come from. It just says "The Lord of the Rings," but a Google search for this card's flavor text has turned up nothing. Also, the art (while not bad) is really plain and I wish it were a picture of her fighting an orc or being rescued from a prison cell in Mount Gram rather than just her standing there looking smug.
(EDIT: As one of the replies has pointed out, the flavor text is a description of Éowyn from The Return of the King.)
Recently I have found a sweet use for this card in my Aragorn / Arwen Undómiel / Frodo Baggins deck - Good Meal synergizes perfectly with Stand and Fight. Consider this: if you lack resources, you can play Good Meal on Frodo Baggins, discard it, then discard e.g. Gildor Inglorion with Arwen's ability to add a resource to her pool and then play Stand and Fight to put Gildor into play for 3 resources. And in a perfect scenario it can be done on turn one. In fact, in this situation you could play any 5-cost ally (belonging to a sphere of influence) on turn one this way, even if you don't normally have access to its sphere.
The Squire's Helm isn't a bad card in isolation. Two hit points are great on your high value low HP allies who you don't want dying because of some surprise treachery or more commonly, shadow effect. I particularly like this with Defender of Rammas which gives a lot more longevity for a cheap but valuable card.
Problem is, since I obtained Hauberk of Mail, I haven't really been able to justify this card too often; just seems more tangible than in a lot of scenarios. And if you're buffing up a high value ally, do you really want to use up your valuable restricted slots on a cheap helmet?