While Protestants and Catholics differ on many points of doctrine, we tend to forget that we have lots in common too. In particular, we can be strong allies in the current culture war. God willing, we will someday become a united flock again, as Jesus wanted for us. In the meantime, we should be honest about our commonalities and our differences.
A particular area of importance for Catholics is about receiving communion in Protestant churches, for example at a friend’s wedding. Please take a moment to read Fr. Z’s explanation here. In a nutshell, he explains that Catholics are never to receive communion in Protestant churches. A Catholic may only receive sacraments in churches that have valid orders and sacraments, such as Eastern Orthodox Churches, the Polish National Catholic Church, pre-Calcedonian Churches and so forth. But even in those churches, Catholics should only receive their sacraments if it is physically or morally impossible for them to approach a Catholic minister and if the reception of the sacraments is required by necessity or a genuine spiritual advantage commends it, and provided the danger of error or indifferentism is avoided.
So you see, there are a lot of conditions here. Even for churches with valid sacraments, you would be advised to refrain from receiving communion in most cases.