Will it cause a rift in the friendship? Will they borrow from someone else and take it anyway, if they take it at all? Seems like a s’fek s’feka.
What would happen if he asked to borrow a bottle of Coke or a can of corn and his posek held that it was mutter to open them l’chatchila, while your posek said it was assur gamur. What would you do?
Would you invite your neighbor over for Shabbos knowing that he is carrying a tissue in his pocket? Or would you give him shalom in the street knowing that he may take off his glove to shake your hand?
There is no end to this and I don’t think you are responsible for his keeping the Shabbos the way your posek determines it versus his posek.