Do bidets help for major messes?

This is a little bit TMI and I like cwillies suggestion. I may try that for my dad if it happens and he's here as his toilet is next to the shower (with shower doors). But what I wanted to say is that, for myself, I put in one of the cheap cold water sprays you can buy on Amazon. And it's cold but it helps me get clean when I am sticky so I really like it. But I'm not old and the cold doesn't shock me at all and it's up to me to turn it on so I know it's coming!. I get the TP wet and I go to town getting cleaned up. The actual spray doesn't do enough to clean me, it takes wiping with the wet TP.

I think that in my next home I will have a proper warm water toilet bidet installed... and maybe still will here as well. But my current toilet is fairly new still. It would take an outlet to be installed but that's not that expensive...

So, if I were you, I'd look into getting it done by an contractor who would install both the electrical and the toilet. But if you want to try just to see if you like the bidet idea, spend a few bucks for the cold water solution. It's easy to install if your husband is at all handy and now I know that's its useful to have clean water at hand.

The cold water bucket next to the toilet was what I did before but it's kind of gross and in the way.

I suggest Huggies as wipes. They are big and thick, unlike those "women" ones.

Does grandma drink coffee or apple juice. If so, you may want to cut them out, I did with my Mom. Try to time when this happens. Maybe you can get her to the toilet before the mess.

Many Bidet fixtures require electricity close to the toilet. (ones that use warm water.) Cold water ones can more easily be attached.
I doubt they would do real well on a "major mess" but they would remove a lot of fecal matter so using a wipe would be easier.
A Sitz Bath might work as well.
Since you know the potential timing for the accident you could set up the sitz bath before dinner and if it is needed it will be ready. If it is not needed the water can be flushed and the basin stored for next time.
(I think a bidet is a great idea and if I ever have to replace a toilet I will replace it with a bidet.)
A few suggestions for you though.
Have you tried getting her to the bathroom before dinner? If she can void then she will be more comfortable during dinner.
If this happens on occasion have you made a list of what you have had that day that might trigger her urge? For some there are particular foods that will trigger a bowel movement.

I like cwillie's suggestion and can go one further by suggesting a "trigger nozzle" hose fitting so you can have it handy in the "always on" position ready to go as required. Having a commode chair can also make accessibility easier.
Something simpler (and a little more 'agricultural') is to keep a bucket of water beside with a rag as a reusable wet wipe (of course you flush the contents after each use) - it also helps to toss in any soiled clothing for a preliminary rinse. For your own sensitivities, it can be worthwhile adding some disinfectant or fragrant oils.

No, they aren't designed to handle major messes, but I'll tell you what worked for us:
The toilet was close enough to the tub/shower that I could use the hand held shower to aid in clean up as mom sat there. This worked so well that I think that even if your fixtures are too far apart adding an extra long hose to your tub, shower or bathroom sink would be worth exploring. There are also bidet sprayers that can be attached to your toilet plumbing but that would be cold water only.