Do you have a contract with the person? If not - all fool you! Get one sorted or replace him/her.
Remember to protect yourself at all times legally and your product/idea. Hiring without a contract can leave you vulnerable - (s)he could even walk out the office and rewrite the code for someone else (or themselves) - what agreement do you have to stop him/her? Sure, you could sue and argue IP rights (though if they recode and not use your code spoecifically then this is very grey anyway - and theyt would probaqbly get away with it) - but it may cause too much damage by the time you get any justice (if you do).
If you DO have a contract, then they are obliged to abide by it (including renumeration) up to the end of that contractual agreement (and any limiting clauses therein - e.g. not allowed to work for competitor for 6 months after leaving project or start up in a competative way - 6 months is the norm in the UK due to precident where long periods were set and courts put them aside - check with your local lawyers for best fits locally).
They are, of course, free to re-negotiate at the end of the contract - and may have escape clauses to force it earlier - you should also have clauses to protect yourself and allow you to replace them or lock them in to their agreement during the contract period.