If you are willing to give her support and encouragement (as in: helping her practise almost every day and keeping in touch with her teacher about how to help her), that will make an enormous difference.
Here's my opinion about the pros and cons of different instruments, if that might be helpful (we are a family of string players).
Piano starts out being easier as you can play little tunes just by pushing the right keys down, and it isn't difficult to hold. It is MUCH harder to get to the stage where you can play with other people, and it can be a bit mechanical, especially if not taught well, as you are not physically creating the sound with your body. I don't think piano would come particularly easily to someone who struggles with reading because you have to learn to take in a lot of information at once, as you are reading several different lines of music at the same time.
Some wind instruments are initially easier than strings, but it is MUCH more competitive to get into an orchestra (4 flutes, over 30 violins), and the chamber music repertoire is nothing like as good or extensive. If you might have access to a jazz or improv group later on, there are opportunities to play wind instruments there.
String instruments are hard to hold and hard to tune at first. Learning to create sound on them is an ongoing process (challenging but fascinating), but, if you have a good teacher and the child and parents are both committed, you can quickly get into an orchestra or other group. The repertoire is amazing and the sounds are, I think, the most personal, maybe because they are closest to the human voice.
Guitar is a great instrument if the child likes folk and popular music, and it is easy to learn a few chords which will go with lots of simple songs. I think there are many guitar teachers who don't teach kids how to strum a few easy chords (which is what they would most enjoy), but teach a very plinky plonky sort of classical-ish guitar playing. I think a friendly guitar playing teenager and a few YouTube videos might be a good way for a 7 year old to start.
Any instrument is extremely rewarding and difficult if you are very advanced; this is just the beginning stages I am describing.