Hi. I can appreciate your dilemma! I come from a non-denominational background, so maybe I can offer some insights into your roommate’s feelings. I doubt your roommate means any disrespect by receiving communion in the Catholic Church. In fact I wouldn’t be surprised if he is completely unaware that the Catholic understanding of communion is any different than your average Protestant’s. In the church in which I was raised, basically anyone who agreed that Jesus was the Son of God, was crucified, and resurrected for the atonement of sins could feel free to partake of communion. So any visiting Baptist/Presbyterian/Catholic/Episcopalian was welcomed and encouraged to partake. If you are not aware of Catholic beliefs about the Eucharist, being informed that you are not allowed to participate can really take you aback!
My first experience with this was when I was seven years old. My family attended my Great Aunt and Uncle’s 50th wedding anniversary and there was a mass we went to before the reception. I remember that when it was time for communion, my dad told my sisters and I that we had to stay seated in the pew. I remember watching everybody else in the
entire church going up, and just our family sitting their all by ourselves. I wasn’t offended, but I was very confused. I didn’t understand what was wrong with us. It did make me feel somewhat rejected. In our church, any of our Catholic relatives would have been allowed to partake of communion. Of course at the age of seven, I hadn’t thought at all about concepts like transubstantiation and the like.
One of the things that first struck me about the Catholic Church when I started researching it a few years ago, was that I could tell there was something different about their view of communion. It may seem ridiculous to you Catholics, but I was honestly surprised to find that Catholics not only took communion every week, but that it was the entire point of why they went to church. (In Prostestant churches, the sermon is the main focus, and in my church we took communion once a month). I also started reading some of the writings of Mother Teresa and wondered why she talked about the Eucharist so much. Anyway, that peaked my curiosity of what the Catholic understanding of the Eucharist was. It has been a
real blessing to me to contemplate this.
Now, (not being Catholic, but considering it) I wouldn’t dream of taking the Eucharist in a Catholic Church. I would feel highly disrespectful for doing so. So basically, I think that all your roommate probably needs is a good, solid explanation of what Catholics believe about the Eucharist. Why don’t you get him a copy of
Catholicism for Dummies? I found that very helpful, and sort of used it as a jumping off point for pursuing topics that interested me in more depth.
One point that I would be sure to emphasize to your roommate is that being Catholic in and of itself does not allow one to partake of communion, but that a Catholic must be in a state of
grace (i.e. confessed and been absolved of any mortal sins). If you apply the restriction to all Christians, of course, no Protestant will have confessed and been absolved of their sins, so it is only natural that they shouldn’t receive the Eucharist either. Realizing that Catholics must be in a state of grace to take communion really drives home how seriously they take communion. (Of course,
don’t forget to explain about the Real Presence, which is at the real heart of the issue.) Explaining to your roommate about mortal sins, states of grace etc. will probably open up
lots of other questions about Catholicism

That’s why I recommend getting him some good introductory material. I also found
Catholic and Christian by Alan Schreck to be an excellent explanation of Catholic doctrines. I really like the Catechism of the Catholic Church, but that might not be your roommate’s cup of tea. You would know best. Anyway, I know that this seems like a kind of touchy and uncomfortable situation with your roommate now, but I believe it will open up lots of fruitful discussion for y’all. I will pray that it will!