No, you should not run refrigerant lines through your property. There are many reasons:
- They will require access to the unit to repair lines.
- A split system, like you are suggesting, has two lines, one of which is often cold. This causes condensation on the line, and if the insulation isn't 100% it's your property the line will be sweating into.
- You can only damage the lines, there is perpetual risk to you, with no benefit.
- The lines being damaged, can only harm you; again there is no benefit.
- The agreement you and your neighbor make will have difficulty surviving new neighbors, as well as complicate the sale of your property.
There is an alternative. Have you neighbor look into a roof top mounted AC stand.
- This keeps the property lines where they are, without encroachments.
- The stands themselves are rather cheap (under $200)
- Access to the AC is a bit more complicated, but not "enter my neighbor's house with a stranger technician complicated."
- A crane will have to lift the unit in place, which typically adds $100 to $200.
However, if anything goes wrong, it's not going wrong in your home. Having something go wrong in your home, even if you had nothing to do with the failure, will put stress and pressure on the neighbor and your relationship. Even if you are completely helpful to the neighbor, there will be ways that you could have been more helpful; and, if you neighbor is pushing for access, even once, you might find yourself in the position of not wanting to help them.