EDIT: Sorry if i'm assuming you don't know to much about the game, you might. But making sure i don't miss something
As for building an army? First know the point level you want to play, there are 3 distinctive point levels that has slightly different rules on what you must and can take. Its basically;
Low: 1 Leader, 2 battleline, optional less artillery, behemoths, optional heroes
Normal: 1 Leader, 3 battleline, optional more artillery, behemoths, optional heroes
High: 1 Leader, 4 battleline, optional even more artillery, behemoths, optional heroes
You can build an army and know the restrictions here https://www.warhammer-community.com/warscroll-builder/
As for Sylvenath, at least she likes the core units, sadly the A+ units are the Kurnoths, but that doesn't mean she has to play with lots of them. You'll want a few Heroes normally as they tend to buff and support other units, as well for you add more units and trees. NOTE: You'll need to pick up some terrain for this army, they really work much better with their terrain, keep this in mind when you are mapping out the cost of the army, 2 of the smaller box sets are perfect for anyone not looking to play serious, you get a full set of extra rules for playing them, like teleporting through them, getting protection, etc...
If you start with 3 Heroes (Treelord Ancient to cast more spells), your Basic battleline of 2 Dryads, a Tree Revenant, that'll be a starting army of 1200, then if you add in one or more of the endless spells (Tho i woulnd't play all 3 of them) that means she'll need about 200pts more to play a 1500pt game, which is a very good starting point. 2k is normal, but many play at 1500 and many that play at 2k will be happy to play at 1500 as well. If she wanted too she can just take 1 Endless spell and play at 1250pts. But you are working with a low model count army, i would go up to 1500 points and add in some more bodies. If you want to go cheaper and not buy 2-3 more boxes of Dryads, Kurnoth will work just fine, 1 unit of anytype is fine, otherwise get more Dryads (tho she might like that).
OPTIONAL: B.c she likes Dryads, If you get a Branchwairth you can summon more Dryads with a spell and those are free points, so keep this in mind as you'll need more Dryads to play this unit, but it is a very strong spell if you play it (Free 2D6
Now about making your army bigger? Get Alarielle and a unit she can summong thats 600pts instantly and she is very fun, also another option is 3 Treelords for the Battalion (its like a formation) thats a fun way to pay them too.