This is an interesting topic.  I was immediately reminded of the movie: "When we were soldiers" (Mel Gibson, 2002).
Part of a dialogue "Mel" is having with his children:
Moore: Cecile? I don't hear you praying, honey.
    Cecile: I don't wanna be a Catholic! I wanna be a Nethodist like Mommy!
    Moore: A-and why's that?
    Cecile: So I can pray whatever I want.
    Older Brother: That's a sin!
    Moore: Oh, no-no-no, that's not a sin. God just made you hard-headed. It's not a sin. Uh, I'll tell you what. You wanna, you wanna pray and thank God for our family?

And he begins with: Hail Mary, Full of Grace, The Lord is with Thee....

I'm sure the Prots would come out with all the 'bible verses' concerning this:(Little do they understand some of that 'bible' they carry, is a remnant of a robbed and looted BIBLE from the Catholics).  
But to the point, many Catholic priests and the writings of the Saints (St. Therese of Lisieux)would also tell us we must also pray to God, in our own words.  Our Blessed Lady has said the same. (For those who want citations, dates, titles etc...don't bother).  

But to address the question...well, all I can say at this moment is: Who knows the mind of God? and to beg the question further: What do we know from Traditional Catholic teaching?

It clearly depends on the prayer and the context of the prayer, does it not?

Quote from: Telesphorus

It clearly depends on the prayer and the context of the prayer, does it not?

Please elaborate on that one.  

If someone is praying in a Protestant assembly they are endorsing Protestantism.

If someone in good faith asks for God's help, that's another matter.  Don't you think?

I think this question is what the VII teaching on "baptism of desire" was getting at. I grew up Protestant, and I would certainly hope that nobody would say that my childhood prayers, full of faith and hope and trust in Jesus Christ, were somehow offensive to Christ. Or that, had I died after nightly prayers, Christ would abandon me to Hell for not being Catholic. However, once I learned the Truth of the Catholic Church, I immediately converted, at least in heart (not 'officially' til Easter Vigil, 2009). Had I not converted, and chosen to remain Prot, then my prayers might be despised by Christ. IDK, good question, lots to think about.

Well, here is a letter from St.Augustine distinguishing what would come to be called material from formal heresy.

"But though the doctrine which men hold be false and perverse, if they do not maintain it with passionate obstinacy, especially when they have not devised it by the rashness of their own presumption, but have accepted it from parents who had been misguided and had fallen into error, and if they are with anxiety seeking the truth, and are prepared to be set right when they have found it, such men are not to be counted heretics. Were it not that I believe you to be such, perhaps I would not write to you."

the only times i heard protestsntas praying

they were dictating longwinded letters to God before audience (family, friends, strangers, passerby)

or they were reciting psalms, usually the Lord is my shepherd which i guess must be the only psalm in the protestahydgat Bible

For a while before I found Catholicism, I went to a "Word Faith" evangelical church. They do in fact dictate to God and actually teach that if they demand something "in Faith", God HAS to obey them! Also you know God truly loves you when you can afford a Lexus. And if you are sick or poor or suffering, you just don't have enough faith to force God to give you health, money and happiness.

speaking of proasugtygasnets I went to steubenville youth conf

oh no nyln thinks hes jim gaffigan

was ten years ago maybe this week for all I know

definately was ten years ago, went in 2002, and it was in july maybe

anyway

we had adoration of the Blessed Sacrament one night

you shoulda see the kids crying and laughing and screaming as the monstrance came around

might have been the diocesan bishop might have been a priest

the bishop did deliver homily at a later Mass so maybe wasnt him

bizarre sight

also during adoration the lay MC had the mic up on stage and kept saying "Jesus" overandoverandoverandover again the whole time

was like "shut the mic off for goodness sakes"

diocesan bishop wasnt there or probly woulda told him to shuddup

later my NO parish priest was quizzed about such things, and he never heard of it in all his years

although he was prob 15 years ordained at the time