“I am a member of a group of people who get together to play cards. I have observed a person who plays cards with us (but is not a member of the group) cheating.
The subject of cheating came up once, and I tried discreetly (obviously too discreetly) to let this person know I observed the cheating. It had no impact. It appears this person is a friend of a member of the group.
I realize there is nothing I can do about the situation except learn to deal with it so that it no longer upsets me. Any advice? — Card Sharp”
You “realize there is nothing [you] can do about the situation”? That is 100% untrue.
If someone is cheating, regardless of who they are friends with, then you need to call attention to it. Either do it discreetly with the cheater and let them know that you know and tell them to stop or do it when you catch them in the act at the card game.