Background: Me and my fiancée have been living together for three years, two in our actual residence. Half a year ago we invited a friend of mine, who was coming to the city we live in to study, to live with us. We didn't set a limit on how much time we would be living together. Said friend expects to stay for at least a year and a half more. We would like to be left alone as soon as the school year is over (three to four months).
There are no boundary issues, or disrespectful behaviour, or anything of the sort. The main reason we want to go back to living alone is that we want our "couple life" back. Many times we want to be alone in our living room, and it would be extremely rude to ask someone living in the same house to stay exclusively in their room, but at the same time it spoils our mood that we cannot spend quality time alone. So there's no real way of fixing the issue while remaining housemates.
On one hand, this will come pretty much out of the blue, though the time limit should be generous enough. When we invited my friend to come over we didn't specify how long the stay would last, and while we expected to last for the current school year, my friend's expectations differ. Until recently we thought my fiancée would have to go live somewhere else for a year for work-related issues, but in the end it will only be about a month. Knowing this, we really want to be alone again.
On the other hand, we know my friend has financial troubles, and right now her income is low and instable. While we have no issues if a payment is late or even doesn't come (since we were already living here and paying the full rental before my friend came), we know that somewhere else that could be an issue. So "kicking out" our current housemate might mean my friend has to leave the city and go back to the family house, and thus would be unable to continue studying.
What we want is to say "it's not you, it's us, so get out of our house" while remaining friends. If staying with us is the only option my friend has to continue studying we would allow the current arrangement to continue, but we are afraid that saying this will mean than only a minimal, token effort is put into searching for alternatives and thus would be fruitless. So how could we tactfully ask our housemate to leave?