How to get out of a mortgage on house that I no longer live in?

You can't get off the mortgage unless your former friend voluntarily agrees to refinance it in his/her own name. The mortgage is a contract among you, your former friend, and the bank; the contract does not care about where you live or the relationship with your friend. A contract can only be changed with the consent of ALL parties to it, which would include your friend and the bank--and the bank will not let you out of the contract, since doing so gives it no benefits but actually harms its interests, by reducing the number of people who have to pay the mortgage. So unless your friend chooses to refinance, you are on the mortgage--unless the mortgage is paid by the sale of the house, that is. (See below.)
If you are on the deed, you are an owner, whether you live there or not. If there are two owners and they can't agree as to what to do, one of them can bring a kind of legal action (lawsuit) traditionally called an action "for partition" in which you ask the court to order that the home be sold and the proceeds used to pay the mortgage (and costs of sale), with anything left over being split among the owners. That is the law's remedy when you are stuck owning property you do not want to: a forced sale.
So you could tell your friend that unless they will refinance the mortgage, you will force the sale of the house; and you can also offer to quitclaim your interest in the home to them as part of the refinancing. (Remember: you are half owner, so you own as much of the house as your former friend.) Hopefully, they will agree to do so, to avoid the sale of the home and because they will end up as sole owner (e.g. when it is eventually sold, they will not have to split the profit with you). But if the don't agree or can't refinance (bad credit), you can bring the action for partition to force a sale.
If you want to explore the partition option more, consult with a local real estate attorney.