Christ’s sacrifice did pay the eternal price for our sins. It made available the graces we need to be justified. The question is how those graces are applied.
Your friends appear to be saying, “The verse says that Christ’s sacrifice cleanses our sins, but it doesn’t say that Christ’s sacrifice cleanses our sins if we do something. Therefore, we don’t need to confess our sins, because Christ’s sacrifice means we are already forgiven.” The issue is that if you’re going to say that, it’s equally legitimate to say that we don’t have to do anything, even have faith – our sins are forgiven regardless of what we do or don’t do. Obviously, no Christian would accept this; whether you’re Catholic or Protestant, faith is absolutely necessary for the benefits of Christ’s sacrifice – justification – to be applied to you.
To summarize: Hebrews doesn’t say that Christ’s propitiation is applied to us conditionally, but that doesn’t mean that there are no conditions at all, as your Protestant friends will certainly attest. The question is what conditions there are, and that’s something this part of Scripture just doesn’t answer.