Is there anything wrong with a civilian wearing a marine pt crewneck from a surplus store?

You’re a civilian? Wear anything you want. Up to and including the Medal of Honor.

Now... if you’re an ethical civilian, you won’t wear anything that suggests you’ve done something or earned something that you haven’t. Obviously, wearing a green beret, or a “BUDS Class 293” shirt is suggestive. Wearing a plain t-shirt, socks, pants, or even a camouflage Gortex coat isn’t necessarily suggestive. You’ve probably seen the videos on YouTube—a guy is wearing just a camouflage blouse AND tries to tell people he’s actively serving, or home on leave. Or, if you were wearing it, and someone tried to thank you for your service, and you thanked them without making any effort to correct them... that’s all bad.

It goes to your intent. If you’re wearing a blouse as a “John Lennon” fashion statement, that’s one thing. Trying to convince people that you served, that’s something else. Wearing an unmarked crew neck shirt is no big deal.

it's a shirt. You're fine. The Stolen Valor Act has very specific guidelines. Your friend needs to take a chill pill.

Not entirely the same thing, but sort of related.

When I got in for the evening at my weekend drill I realized I FORGOT the green t-shirts that I wear under my ACU. The ONLY option I had open to me at the time was to go hunting for one that evening, at what was probably a local Walmart.

I found an acceptable green shade of shirt for my ACU. Unfortunately, it was plastered with USMC!!!!!! across the front.

Well, you gotta do what you gotta do.

Then later during the battle assembly they started calling out names for the monthly urinalysis. For those in the know, it requires I take off my uniform top exposing my t-shirt. This would be a funnier story if my name was called, but it wasn't.