"Invitation is from another youth with Protestant Christian Evangelical parents. Whatever that means."
Aw, I loved that phrase!
My opinion? ABSOLUTELY YES! Don't worry, the child will probably enjoy themselves! Christian churches are NOTHING like what you've experienced as a JW. Some are kinda fun and have neat activities for teens - often in the line of volunteering which is GREAT for the community and strengthens the child social skills.
How do I know? I was confirmed Episcopalian, attended a Catholic high school and university, Presbyterian middle school, have Baptist friends, and enjoyed Jewish summer camps and services at synagogues. NONE EVER REQUIRED MY ABSOLUTE LOYALTY. Your kid will be fine and will get to be around other kids doing normal stupid teenager things. Seeing the variances will both educate your child AND teach them to think! (Isn't this the goal?). There really is nothing to lose. :) Assuming the child really goes (remember, you have a teenager - they do dumb things), appreciate the fact that they're hanging around kids whose parents actually attend church services!
My 21 year old experienced everything from churches to synagogues and has a huge respect for all faiths and belief systems. Might be just me, but I wouldn't have it any other way. :)