Gittin 61 - Baking bread
A woman can loan to her friend a sieve or a hand-mill, even if her friend uses forbidden produce of the seventh year, because her friend may use the implements for permitted produce - but she should not cook with her.
One can deposit his tithed produce with a friend who is not careful about tithes, and should not suspect that the friend will substitute it. But what about the rule that one should tithe flour when he gives it to his mother-in-law and also after she bakes it into bread? His mother-in-law would substitute burned bread, to help her daughter.