WOW, I took Cascara Sargada for the first time a few eeks ago, and I will never use Magnesium again.
This is hard to find, because it is so good I guess.It acts on the liver, producing bile salts, the same action the eating a meal will produce a Bowle movement.
I just take two teaspoons full, and boil that for a few minutes, then let it steep for about an hour, and drink the tea. It tastes bitter, mix with honey amnd drink when cooler so you can swallow faster.
There will be other effects,as in a better bowel movement, lots of old hard stuff. She will feel very good I bet.
Can mix in a liquorice stick into the tea brew. And many other herbs are available to aid the process, but Cascara Sargada tree bark made into tea yourself [not a premade thing, please!] is the key SHE MUST TRY.
Good luck finding it... I have a store here that sells it, they found it in the ordering catalogue they had [I do not know which catalogue, ask your health food store].
The active ingredient in Milk of Magnesia is the "Magnesium Hydroxide", as you know. Other magnesiums laxitives are similiar in how they work.They will make her lose more weight, so try to get her to try others,esp, herbals, and rotate them if necessary.
Lots of people do what she has done, its not ideal though. Everything in its time: she has been waiting for you to help her find a new path, but you tend to respect her so much.....Its time!
Tell her how you feel about this situation, starting with "I love you dear, and we need to talk...