You definitely should draw up an agreement that clearly spells out that you are the owner, conditions of return, and to make sure you are covered in the event of damage or theft (left in a studio open to the public, there’s always a chance someone may break in and steal it).
You also might want to consider charging a usage fee if there is any way already in place to track the # of hours that the GF is cutting or engraving. It does not have to be a high rate, but if the person has the machine for year in heavy use, it is possible the tube will be spent by the time you get it back. That’d be an unfortunate way to break the news to the family. You bought a GF and let someone use it for a year for free, and now you need to buy a new laser tube before you can use it yourself.
If you Google “equipment loan agreement” you can get some ideas.