To Butt or Not to Butt In

Here is a quandary of which I have been considering since yesterday and would like your opinions... 

Yesterday I needed to run some errands, which included stopping in a local big box retailer. While I was walking up to the entrance i passed an elderly woman. She was moving very slowly supporting herself with a rolling shopping cart. Clearly suffering with kyphosis, she appeared to have related coordination / mobility issues. Never the less she was maintaining her independence, doing her own shopping without assistance. Her attire was a simple sweat shirt and sweat pants; the elastic waist pants were beginning to slide down. As she leaned over the cart a crescent of skin had been exposed. Walking past her I thought to myself, "she surely would be embarrassed by this display but would also be embarrassed if I mentioned it to her". At the entrance she sat down in a motor shopping cart which effectively concealed her attire malfunction.

They intend to make a spectacle of themselves.

Coincidentally as I was departing the store, she was  directly in front of me. She was waiting in line, again standing and supported by a cart. Unfortunately   her pants had dropped low enough for a considerable amount of her intergluteal cleft to be on display. It was a lot of skin and the exposure was progressing as she moved forward. Plumbers and Vegas showgirls would have been embarrassed to make such a show of themselves in the checkout line.

Without really thinking I came up behind her, as gently and discreetly as possible grasped her waistband and pulled up her trousers, saying quietly "I just know you don't want to lose your pants." The elderly woman nodded but did not look at me nor say anything. The sales-check did glance at me and mouthed "Thank you".

Did I do the right thing to intervene or should I have minded my own business? In such a situation would you want people to ignore your problem or assist you?