Letting a Room to a Friend - Conclusion
If you let a room to a friend (any friend, not just a vulnerable person) with all the best intentions, but then find it's not working out, and you need them to move out or they feel compelled to leave before they're ready to, you are not helping that person, or your friendship at all - after all, if they can't depend on a friend to give them a stable home for a time, they might be entitled to ask who they can depend on?
To conclude: never offer to let a room on a whim, think long and hard about the circumstances and your own motives, just as you hopefully would if you were letting to a stranger. Always have an Agreement with House Rules or at least a very clear understanding (preferably in writing) with your friend first, and decide whether it's a contract to rent (i.e. you're taking a rent, which even if it isn't near the market rate, is still way above the lodger's expenses or the lodger is doing several hours work for you in lieu of rent) OR your friend is simply staying for a while as a guest (just paying you for the additional utilities and possibly a token amount so you're not out of pocket).
Remember, if you do accept a rent which is above this, you are bound by landlord's obligations, and could find yourself on the wrong side of the law or at least in hot water if you don't honour these!
Let to your friend, don't let your friend down!