Speaking as a grandfather who regularly uses mdma my thoughts are as follows:
The mdma experience is just as beneficial / enjoyable for older people as for the young.
Re health you should ascertain whether grandma has no serious heart conditions and that her liver and kidneys are okay.
She should of course be aware and willing to try the drug.
You should start her off on a half dose. I would suggest about 50 to 60 mg.
You can always give her a litttle more if everything is okay after about an hour.
Don't forget that women are more affected by the drug than men. Unless she is a very big lady I would think that a dose of 100 mg would be about the max she could enjoy.
What concerns me more is what she could do during the roll. She too will feel luvved up and long to reach out and touch people. For her the ideal would be to roll with one or two people who are close to her own age.
She might like to listen to music, but she might prefer to hear music that she is already familiar with rather than house, techno or urban.
Lights, would you put on candles or arrange some special lights?
Would you put out cushions, so she could lie down if she felt like it.
What would she wear? Something loose and easy would perhaps be best.
Have you thought that she might want to touch herself or be sexual in some way. How would you deal with that?
And finally are you prepared to hear the things she might confide in you?
In other words I think that getting your Grandma to roll is a great idea, but that you should find somebody else to supervise the first roll.