There is a lot to know, and the product quality greatly varies.
First off, check out sportbiketrackgear.com and look through their helmet selection. In most cases they do video reviews which are full of the details you need and without the fluff you don't.
Here's the criteria you need to search for:
Fit: Many brand's models will have multiple shell sizes. The more shell sizes, the better for a custom fit. Cheap stuff (like say HJC) will use the same shell for small, medium and even large as well only changing the shell for XL and up. Whereas a brand like Shoei will usually have 3-4 shell sizes so someone wearing a small isn't carrying around a large sized outer shell on their noggin. Scorpion is also good about this for a medium pricepoint brand.
Weight: Heavy is a pain in the neck. Literally. Sometimes we sacrifice because we want inner flipdown sunshields and whatnot, but in general you want as light as you can get. Cheaper stuff is generally heavier, it costs more to produce a lightweight helmet.
Safety Rating: DOT means absolutely nothing. The helmet you buy that says "DOT" on it wasn't actually tested
. This is your squash we're talking about here... protect it right, go with a Snell Approved helmet. Also ECE is a step above DOT (and both will also be DOT is sold in the USA because Snell and ECE go above and beyond the minimum DOT requirements)
Shell material: You want composite. You don't want fiberglass. Strength and rigidity is paramount and composites are the way to go.
Airflow/Ventilation: Let's face it, full face helmets are hotter than half-shells. But one will save your life while the other does nothing more than hold your hair in place... for this convenience, you must sacrifice. Watch all the review videos you can because they will always discuss this. My Shoei RF-1100 flows AWESOME, almost feels like I have a series of fans inside the helmet when I am riding. My Scorpion EXO-900 flows "decently" but there is a noticeable difference. Cheaper helmets will flow less, almost always.
Sidenote: They all claim anti-fog faceshields but its bullshit... they all fog up and you'll be venting them at every light, or riding with it slightly vented anyway. The cool thing is now you don't need sunglasses, you either buy an extra shield thats tinted, or buy a model that uses self-tinting photochromic shields that tint and un-tint in available light. Bell makes four models that use this and Akuma makes one that does.
Quietness: If she falls in love with a particular helmet, it fits her head perfectly and fills the other requirement... just wear a good set of earplugs. It's not so much the exhaust, but the wind buffeting that can seriously damage your ears. Higher quality helmets will be quieter, cheaper gets much louder.
Aerodynamics: Nothing worse than doing a headcheck and BAM the wind nearly rips your head off because you turned it sideways. Yeah, it sucks.... be sure to get a quality helmet, they will be tested for aerodynamics.
While in SOME rare cases you can spend $200 or under and get most of that, it's the exception rather than the rule. Quality means a LOT of research & development, expensive components, and higher cost of labor. Helmets aren't cheap. One way a helmet can give you most all of the above but still be cheap is by cutting corners in the padding. HJC make a decent protective lid... but the insides break down by the end of the season, so you're buying a new helmet each year. So that $189 HJC will cost you $945 over the next 5 years. That $489 Shoei/Arai/Shark/whatever will cost you only $489 over the next 5 years because it is made of better quality components.
Fit can be tricky. She may be say a small in the first ten helmets she tries, but a medium in another brand. Even within the SAME BRAND, different models might change her size. Find a BIG shop that carries a wide variety. Try them all on, write down which brand & model fits in what size... then shop online for the best price. OR find a good e-tailer like sportbiketrackgear that has a killer exchange policy, and you'll have no worries.
Once you've owned a QUALITY full face, you'll never go back. No wind, no bugs, rain doesn't sting, and you know it'll be as safe as it can possibly be. Many folks who detest full face helmets did not use a high quality one.