Is my friend obligated, according to Sharia, to return the money to me, based on his guarantee to the partner who turned out to be a swindler, especially since I did not directly engage in business with him?

If the friend guarantees the capital in mudarabah absolutely, whether due to transgression or negligence on the part of the mudarib or not, then this guarantee is invalid. However, if he guarantees it in the event of the mudarib's transgression or negligence, then the guarantee is valid, and your friend becomes a guarantor of the money.

So, if the mudarib betrays the trust and squanders the money through his transgression, he is liable for it, and your friend who guaranteed the money to you against transgression is also liable for it. You are permitted to demand it from him, and he is obligated to pay it to you.