Poor boy! I think he's really cute and her appears to be supremely tolerant of whatever that lady thinks she's doing.
I was looking through more videos and it appears like she/someone is training him to rear? And he's considering doing it undersaddle as an evasion?
I've dealt with that kind of rearing with Lacey, those little "I'd like to rear if you don't stop but I know it's against the 'rules'" ones and even though she doesn't do it anymore, it's always an "option" for her. I can't ever push her very far because if I do, she's going up. It's a real pain.
Personally, I'd pass on this guy as a dressage horse. As a project/"rescue," I'd take him if I could.
Here's the video of him rearing:
And the one where he's evading whatever she's trying to get him to do by hop-rearing. :(
ETA- I did see a video of him at liberty and he looked SO much better. Stretching his neck down, relaxed gaits, a little playful but not disrespectful, he looked like a real sweetie without a rider! Lol