Do you have any training as a Nail tech because ideally, you still need to do a Trades test. Otherwise, can you find a friend to help you out?
You want to check that they have adequate qualifications (not a 1 day course) with certificates and are fully insured, use professional brand products and have plenty of experience. Check the paperwork yourself, don’t rely on them saying they’re qualified and insured.
Remember that even though they’re a separate business, if they don’t treat clients well, it will reflect back on your business. Make sure you give them a written contract so you are both clear about what the agreement includes and consider adding a 3 or 6 month review into the clause to enable you to easily cancel the arrangement if it’s not working out well. Ensure you have a clause covering absences so they understand that rent has to be paid regardless of whether they’re in situ or not.