Catholics Taking Protestant Communion

I e-mailed a Catholic colleague of mine this question:

If a Catholic believer takes Holy Communion offered by a Protestant priest, is it, from the Catholic perspective, a sin, or simply an ineffective sacrament?

This was his response:

Good question and there is some debate around it. Not so much that it is a sin, exactly, but whether it is appropriate to participate in a ritual established by Christ that is seen as not bearing the fullness of the sacramental sign.

I say if the church is open to Catholics receiving (so many are "any baptized believer is welcome"), and the person receiving does so with the understanding that it is a ritual of love, remembrance, and the presence of Christ among us, then it is fine. Others will say that this is the one place where the rubber hits the road in the brokenness of Christian communion, and we should be "separate" in action until unified in belief.

I bend toward option #1. But I can see it both ways.