Obviously, there is a lack of trust in your relationship, but that would be understandable, given her history of using heroin and your fear of a relapse. I am reluctant to make suggestions about the worse case scenario but given the evidence, your enquiry and concern, I am going to make a suggestion of the possibilities.
Firstly, it is not unusual for some people to put foil in a pipe. She could be using it to smoke hash or weed because the hole is too big.
Secondly, she uses the pipe to smoke crack by placing some ash on the foil and lightly melting the crack on the ash with a flame. She may also be using mini spliffs to smoke heroin, as a heroin roll-up is fairly common with some people. Crack and heroin go together for many.
One of the reasons I am speculating is the fact you are suspicious. That must count for something. However, whether you are being needlessly suspicious or otherwise, I hope you can resolve your relationship problems completely in time. Being overly concerned about what she is up to is unlikely to help her avoid a relapse but developing an honest relationship would help in the long-term.
