Not a very nice situation for all involved. 10 years ago I worked as a Healthcare Assistant at the Royal Free and helped to look after many people with skin problems from elderly people with ulcerated legs to young people with severe psoriasis.
All these conditions are very upsetting for the patient and whilst they can be " offensive" to the eye and nose it is so important not to show any disdain as the patient will already be very aware of the situation.
I'm sure this lady and her son were very conscious of the smell and must have felt embarrassed.
I also would question whether she should have been allowed to travel if her condition was as bad at the time of leaving the UK. But of course it could have flared up during her trip and in that case she would have had very little option but to get home asap.
I think that a little superficial cleansing of the area before boarding or actually on board could have helped but I also realise that Crew aren't nursing staff and some wouldn't relish the thought of assisting in this way.
Ideally there should be nursing staff at airports to tend to this eventuality but of course in the real world this just isn't practical.