Here's some basic info about Angelman, and some credible resources with which to follow up.
There was a child with AS in our speech-language clinic when I was in graduate school, but she wasn't my client, so I only really know the broad strokes. She was largely nonverbal, and her clinician spent months just to get her to imitate signs, though people with AS generally understand more than they can express.
If you have worked with children with various disabilities before, the best advice I can give is that it will be similar to taking a little bit of autism, a little bit of ADHD, and a little bit of physical disabilities, and utilizing/adapting various techniques and strategies to fit the child with whom you are working.
Firstly, if the child has seizures, you need to really make sure that this is under control prior to doing any mounted work. As always, make sure there are no contraindications for TR. For any physical concerns, you can consult with the child's OT and/or PT, in addition to parents.
I'd make sure that you were working with a side walker who was super attentive because of the lack of coordination and impulsivity. You don't want to pair the child with one of those side walkers who spends most of the session gazing at the other riders.
Since verbal language is often severely compromised, pair brief directions with visual cues. Ask parents if child already will do some signs - learn those from them - and also you'll probably end up creating your own signs for things like "hold the reins", "whoa", etc. Always pair these with your verbal directions. In addition to the sort of pseudo-sign language, you can also create simple pictorial or other visual supports to use during part of your lessons. (Some examples: I have used a red on one side/green on the other "stop" and "go" sign. You can also get these really big, soft dice that the child can throw from the back of the horse, you and the child count the number the dice landed on and then the child and the horse take that number of steps. Go through TR catalogues but you can also get tons of ideas by looking through catalogues that make products for Occupational Therapists, School Psychologists, Physical Therapists, and Speech Therapists that, with a little creativity, can be manipulated to work in your lessons.)
Divide your lessons into a few parts: Don't spend 15 minutes working on a half seat. The child will lose interest quickly. Split it up into a few brief "mini-lessons". You will want to model the task for the child and make sure stuff is pretty interactive and engaging. Standing in the middle of the ring and giving a lengthy string of verbal directives will not be really effective. You can revisit any of the "mini-lessons" later in the lesson or in subsequent lessons because the familiarity and repetition will help the child eventually retain the learning.