If it will hurt the grandma physically or financial then I think I might attempt to sit down and discuss it other wise I’d stay out of it. More often then not the truth does come out, it may not come out as fast as we’d like but eventually it does.
The slip side of that is, more often then not when we disclose something about someone else to someone else, they don’t always want to believe it and we may damage our own relationship with them.
I think if you are able to disclose a little more of the circumstances involved you may get more feedback on how to handle it.