My boyfriend is ferrying another girl around, what does that mean?

I am in a relationship with someone I genuinely adore. He is a marine and lives at a bachelor’s barracks with many other ‘single’ marines. However, one of his friends has a girlfriend (which I refer to as ‘S’) living with him in the bachelor’s barracks. My boyfriend is casually trying to help them out.

He accomplishes this by ferrying her around to and from her job at subway. As well as, loaning her his 2008 camry, his prized possession. I approached him on this subject as the friend has a car he could loan ‘S’, his response “she does not drive stickshift”. He is also accepting her calls and talking to her in front of me for fifteen minutes while we are on a date. He says it’s to help out his friend…

Just for background information ‘S’ was raised in a military family (her father is a gunny sergeant) and prior to this relationship was in another relationship with another marine and acquaintance of my boyfriend. She lived at the barracks with that marine as well. Is it possible that she is looking to settle down with a military man? Perhaps keeping her options open towards my boyfriend?

Or, is it a cultural thing… I could use a little perspective, I don’t understand military relationships. Is it possible that this is a networking position for my boyfriend’s career?