My take on stealing/disrupting/destroying pieces of soul resting with someone else, would rest on the idea that it is a two-way street. You become friends with someone, help them in time of need, it costs you a bit of soul to go to certain lengths. It restores theirs. And the other way around. Bob says that all sorts of things can restore 'soul', from a hug, to someone saving your life. (Along those lines, at least.)
So, you are bestest friends with someone and an insindious mage somehow alters the piece of your soul with him. You still have some of his soul, by that reasoning, and it wouldn't be altered. It could be healed through time and effort. As such, I'd rather see it as dimming that part of the soul, unless you somehow manage to erase both of the exchanged pieces of soul. That in turn, would clearly be altering the mind, as it is counter-intuitive to stop liking someone just like that.
For story and/or game purposes, however, I wouldn't even go there. The best you should be able to do by altering 'borrowed soul', is make someone grow colder towards someone else, if you don't want to affect their mind directly and it probably isn't worth the effort. Any individual with a bit of self-reflection would simply assume he/she had an off-day and apologize sometime later. Simply removing memories of friendship altogether, would truly destroy the borrowed soul, but obviously be Lawbreaking.
Another question you might wish to consider, what good would it do you to alter someone's soul, wherever it might reside? Bob noted that humans always get so worked up about their souls, while it isn't that much of an issue. Besides, beings like the angels,
supposedly composed of 'pure soul',
might detect such magic and object to it. It might be an interesting hook, but I would make sure it is never considered worth delving too deep in the matters of the soul, as the mind hovers so close-by.
My €0,02...