That does not sound like a sexual harassment accusation to me at all and it sounds like he's taking a pretty minor interaction or miscommunication way too seriously. If that's exactly how things happened, he likely didn't do anything wrong or untoward and he shouldn't really dwell on it. It's also probable that she didn't mean it in a "sexually assault" way because that would be a crazy leap. Maybe she just didn't want the help and maybe she takes offense at everyone else trying to help her when she's able and that's how she worded it. We don't know, because your husband didn't ask for clarification. It's possible that she saw it as a come on, but opening a door for someone just isn't, so it's unlikely or she was wrong to think so.

I've also worked retail and have dealt with handicapped people and I learned to ask if they'd like a hand or some assistance. I get that everyone assuming you can't open a door when you're in a wheelchair can get tiring, maybe that's what happened here.

Either way it's small potatoes, maybe ask him if he's feeling alright. It's possible there's some other stressors or another unrelated thing that happened at work and he's processing this minor interaction in this way because of it.