It seems to be a play issue, or battle of wills maybe, not training since 90% of the time, he's fine. He's not ever done that to my husband, although hubby doesn't ride him much or often, and only in the round pen. They just tolerate each other.
WEll then it may be something your DIL will have to learn to deal with. IMO, every horse has a hole, a problem. Once you find the hole, you just have to decide "is it something I can/want to deal with.
Since it's not often may just be his hole:)
Becky