My cat kept a string in his stomach for over two weeks and proceeded to puke it. He was doing fine otherwise, eating, pooping, acting normally.
If she thinks there might be a pen in there, I'd look for abnormal stools, vomiting, lack of appetite, lethargy. IF any of these signs show up, I'd be at the vet asap. I mean, I wouldn't be surprised that if the dog did swallow the pen, that it's somewhere jammed in one of the many folds of the intestine, pens aren't exactly flexible, and at the same time they're thin so food may easily go around it, but eventually the intestines would get inflamed and irritated, and that's when you get clinical signs.
It's worth doing an x-ray (even if the pen doesn't show (and I assume it has some metal in it so it probably would) you can see gas and accumulation of food, gives you an idea whether things are right or not), and it will be more expensive to wait for a clinical sign and possibly go for an emergency exploratory surgery in the middle of a Sunday night when the dog starts to crash...
Your friend has to think of an x-ray as an investment, not an extra expense. What if the pen isn't in there? Well he/she just paid for some very nice peace of mind, and can finally take a deep breath.
“Your birth is a mistake you'll spend your whole life trying to correct.” Chuck Palahniuk
I love pus but I hate people.
I can say words like undifferentiated gonads now!