Sorry to hear this OP.
The whole situation must incredibly stressful and worrying.
Her behaviour definitely seems extreme and most concerning are the elements that result (either by design or as a "byproduct" of her behaviour) in him being alienated socially - especially as the social engagement students experience at Uni is a significant part of the whole experience.
It must be very difficult waters for you to navigate.
Personally with regard to this event I'd be inclined to stand my ground - simply because there are some very solid reasons for doing so.
The recent bereavement means it's especially insensitive to put pressure on the host to include (at their expense) someone they have never met.
Aside from the request being rude, I'd also point out that, in the circumstances it's far from the best opportunity for your DS to introduce her the the wider family.
Does he want her to meet his Uncle under the cloud of him being the pressured to do so at a clearly sensitive time? If he wants her to be accepted by the wider family you can reasonably argue that the timing could not be worse.
Wider issues aside, I'm not sure what else you can do.
My worry about contacting the Uni would be that he (and she) could perceive this as you being interfering and controlling - thus strengthening her position.
Sadly I think the realisation that the relationship is toxic needs to come from your son.
I think all you can do is stand you ground when the situation to do so presents itself (such as this event where that rationale for her not being included is very strong) and where possible encourage him to pursue other interests.