Have you considered or tried leads that attach to the collar using a French Scissor snap? (Like the one in this listing from Ray Allen - http://www.rayallen.com/ramCart/cartFrame.htm?cartDetail.php?prodID=386~mainFrame - the one on the left.)
Those snaps are fairly large (may be weird looking on a Yorkie....) and they are very easy to work. You push down the sides to open it, and just let go and it closes. Even with limited hand strength and dexterity, your mom should be able to work this one.
I don't know of anyone that makes a "small" version of this, though. Most the leashes Ray Allen sells with this type of snap are more geared toward larger dogs.
Those snaps are fairly large (may be weird looking on a Yorkie....) and they are very easy to work. You push down the sides to open it, and just let go and it closes. Even with limited hand strength and dexterity, your mom should be able to work this one.
I don't know of anyone that makes a "small" version of this, though. Most the leashes Ray Allen sells with this type of snap are more geared toward larger dogs.