I don't quite know the lingo, so apologies for that:
The Joker was on the board in place of a black 2 - I replaced it with a black 2, and I know that I have to use it with 2 tiles from my hand to make a meld. I have a red 5 and a red 8. There is a red 7 elsewhere on the board. So I did red 5-J-7-8.
Technically, two of those tiles came from my hand. However, I was only able to make the meld with the red 7 on the table. My mom said it was against the rules, that you needed to be able to use the two tiles in your hand with the Joker to make a complete set.
My red 5 and red 8 alone couldn't make a complete set with the joker...but technically I did use the Joker in a new set with 2 tiles from my rack. So which of us is right?