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.
Great post, AugustaB!

You put out some excellent questions for the OP to think through before discussing MDMA with his grandmother. And I totally agree on the recommended dosage: assuming she's in pretty good health, 50-60mg sounds about right for a first time.
OP, I think it's great that you're so empathetic and care about your grandmother's emotional quality of life. If she decides that rolling isn't for her, I hope you two can spend some time together doing stuff that she's maybe not had the chance to try yet. Or stuff that she didn't do during her marriage because healthy relationships involve compromise.
I bet she'd be really open to new experiences. If she's open-minded and intellectually lively, she'll welcome trying new things and meeting new people, especially people who are passionate about life.
My grandmother developed a love for baseball at around 70 or 75. She'd worked her ass off her whole life and never had the opportunity to do stuff like that. (And her family is really sexist. Baseball games are FOR MEN. ONLY. PERIOD.) So my mother sure as hell wasn't going to give up a few hours to sit in the bleachers at the Oakland Coliseum, doing something that was totally boring to her. (That's my mom. It's all about her.)
So, at like, 75 y.o., my grandma took the train to the stadium by herself and checked out a game. Chilling in the bleachers with the loud mouths, my favorite section too. She had a great time. As soon as I found out, we started going to games. This was a totally new experience for her (and me) and we went to games all the way up until her last year: 90. Those are by far the best times I've ever spent with any of my family.
So even if rolling doesn't work out, go out and experience some new things with her. I bet you'll both enjoy yourselves.
If you do roll, I just wanted to emphasize one of the things AugustaB said: be prepared to have some intense personal conversations. Grandparents were just as crazy as we are, maybe more so, because society was a lot freer in some ways.
I understand why BLers can tend to be ageist sometimes. 40 seemed pretty damned old to me when I was 22. It doesn't seem so old now that I'm here. But a lot of older people, say, 60 or so, can be a lot healthier than younger people.
At a certain point you realize that you can't just fuck around and rely on your youth to get you through the day after a hardcore night. So you start taking better care of yourself. Don't assume that every 60 year old you see is ready to keel over if you blow at him. There's a great clip on YouTube of this 85 year old guy (ex-Marine maybe?) beating the shit out of a 20-something thief, using the sweet science. He fucking destroyed the guy.
Personally, I'm pretty motivated to live as healthy as possible so that I can still roll and trip at 65 and beyond. One of the good things about aging is that a lot of the crazy-making emotional shit fades with time. You get better at dealing with it and you stop caring so much about minor bullshit. So less baggage to take on your trip.