mw13 – You are correct that there is a distinction between an issur that he can do with or without your assistance and an issur which he needs you to enable him to do. This is in regards to lifnei iver.
I believe that ‘mesaya liyeday aveira’ applies even when he has the capabilities, but you are making it more convenient for him.
However, this distinction is not unequivocal; in a situation where you are creating a ????? which wouldn’t exist otherwise, there is a problem of lifnei iver.
There are at least two sources for this: 1) Lending somebody money without proof of the loan is assur, because the borrower may deny the loan. 2) A father may not hit his older son, because the son may get angry and retaliate. The son can hit the father at any time, with or without the father’s instigation, yet it is assur because you are creating a situation of ?????.
This plays a role in issurim where the person wouldn’t do it [even if he has the capabilities to do so] without your involvement.