I have mixed feeling on this subject.
I was an Episcopalian, and I still have a number of friends in TEC. I certainly have no reluctance to pray with them–Grace before meals, intercessions, and certainly the recitation of the ‘Our Father’ when appropriate. Many of the Rites in the Book of Common Prayer and the *Book of Occasional Services * (such as ‘Compline’) were co-written with Catholic groups, or, at least, written to conform with Catholic practice. While I will not receive Communion in an Episcopal church, I would not hesitate to attend a ‘Service of Advent Lessons and Carols’ or similar occasion. I also have gone on Sundays (in addition to Catholic Mass) if a friend of mine is scheduled to preach.
However, I have noticed that many of my TEC friends have become stand-offish, since my Reception into the CC, and a few have expressed that they are offended by my refusal to participate in the Eucharist.
I think it comes down to a matter of degree. Occasional attendance, as an observer, in the name of Christian fellowship, would seem ok, but doing this on a regular basis is something else again.
In any event, I always make mention to my pastor of my plans. He has never raised objection.
BTW, I always extend an invitation to my Protestant and Jewish friends to join me at Mass on Sundays, or Holy Days (or other Church events). Not one has ever taken be up on it.