Do they let it sleep in their bed and give it a nickname and call it best friend? run....
Do they struggle against it and want so much to let it be gone but just can't get rid of it? then that's a different story and maybe you can help but consider:
Is it something that will affect your relationship as b/f+g/f finances, newlyweds. spouses any point down the line? then consider hard if you want to live with it too because it will be your struggle to help him/her with that struggle
would you be willing to fight against it with him/her? cause they will likely seek your help to overcome it if they can't do it on their own strength and they aren't trusting God to break it, or if it's not something that they alone in their power and God's can break, it will be your problem
How much do you love that person? enough to deal with their problems?
what nature are these problems in? sexual, fiscal, attitudes, actions, thoughts, etc? he/she will have to get past them or that person and you will struggle with them all the time you are together.....
How much does it matter to you and to them that they break free of it? enough to sacrifice anything else at all?
just some thoughts, sorry I don't have any hard and fast answers except for if they own it and make it their own and don't wanna let it go run away from a relationship.... otherwise, depends on what it is, what they mean to, what you mean to them and what the sin in question is....