On this topic, I would tend to empathize more with Eric's reply.
Under certain circumstances, attending any non-Catholic church or denomination can weaken your faith because what they believe about Jesus and His Church is different, sometimes very different, then what we believe as Catholic Christians.
We are not in a Common Union with Protestant Christians.
Friendships and interacting in fruitful faith-sharing dialogue (where we both share our faith differences and similarities) is good and should be encouraged. Like having a weekly pizza party to discuss faith issues . . . maybe a different topic per week.
That said, when we attend someone else's non-Catholic faith services on a regular basis, we are implicitly saying,
There is something you have in your congregation that I am not getting at mine.
Here we have to separate Teachings from practices and activities. Someone may belong to a weak Catholic parish where there is little, to no, parish activity or anything to challenge the mind and intellect. For Catholics in this situation, they should work to be that source of activity which is lacking at the parish level.
Even though you may feel more inspired, challenged, and motivated to spread the Gospel in an active manner at a Protestant congregation, you can't lose sight of the fact that you are believing less and implicitly rejecting more as a Catholic when you attend a Protestant worship service.