I know you said you'd feel weird asking the mom, but I think it'd be a great ice breaker to get to know her a little better. (Then again, I am on the shy/reserved side when it comes to talking to people I don't know, so I am always grateful when I have an "excuse" to start up a conversation. Otherwise, I'll wait for them to speak to me. Lame, I know.) When you RSVP, you could just mention that you were wondering what are some ideas of things she'd like since it is your son picking the gift.
With that being said, our 5-year-old really does enjoy her Littlest Pet Shop as do her classmates (one day, her friend showed up on our doorstep with a suitcase full of "pet shop" stuff that she and her younger sister had drug through the snow so they could play--didn't have the heart to turn them away even though we were leaving for errands in a half hour.) I like the carrying case idea and the new triplets--very cute.
But since you were looking for other ideas, our daughter (who is a tomboy but still) likes:
My Little Pony
Outdoor Stuff--sidewalk chalk, bubbles (wants those 3-D, glasses ones

), jump rope, bouncy ball, gardening stuff
Toy Animals--Schleich (?) brand
Build-a-Bear
Puzzles and Books
Arts and Crafts
Lip Gloss (may or may not be okay with the mom) and nail polish