This is a darn interesting question because it's not always easy to figure out what songs are on what movie soundtrack. There are also two other complicating factors: movies also have scores and there are often songs included in the soundtrack that never make it to the cinematic release (usually due to the editing process, the portion of the film with that song ends up "on the cutting room floor", as they say in the biz).

It's also sometimes just about impossible to catch where in a movie the music appears. Sometimes it's music playing on a car radio for just a few seconds, sometimes it's overheard on a bus the character's riding, and sometimes the music is reserved for playing during the closing credits. Other times it's central to a portion of the film and some films are partially developed around specific musical sequences, from the obvious (Vincente Minnelli's wonderful An American in Paris, written around the Gershwin songs and lyrics of the same name) to the more subtle (Wang Chung in To Live and Die in LA).

Back to your query, though. You could dig around in Gracenote, but while their database has the information we seek (song = "Born in the USA" AND title contains "soundtrack") their search facility doesn't let us do sophisticated two-field searches.

Fortunately IMDB.com comes through, as it often does. You need to hunt for it, but the site has a "soundtrack seach" capability you can find here: IMDB soundtrack search.

Now we'll do two searches, one for Born In The USA and one for Love Me Tender.

The results: you're right and she's wrong. In fact, Born In the USA had made it into less than a half-dozen films, notably including Canadian Bacon, Mask, and Skins. By contrast, Elvis' huge hit Love Me Tender appears in almost twenty movies, including Backbeat, Blue Money, Die Hard 2, FM, Honeymoon in Vegas, Porky's Revenge, Princess Diaries 2, Touched by Love and Wild at Heart.

There's probably a custom search engine somewhere that makes this kind of search easier, and lets you screen out the TV show matches, which IMDB can't do, but even with its current capabilities, you can definitely figure out the winner of this bet.

Now, a tough trivia question: what song or songs have appeared in more soundtracks than Love Me Tender?