There's a girl flirting with my man, do I have the right to be upset?

My man is in the army, he's a private, and his squad leader had a bad breakup recently, and he talked her through it, telling me that he'd rather she lean on him for a bit than to have a bad fling and be taken advantage of. This was going on the week before Christmas (as far as I found out), and she's been flirting with him since, asking him what he wants her to wear for his birthday and other things like that. On his birthday she went and took him out to lunch, and then when he got sick a few days later she went and brought him soup. My love and I live about an hour and a half apart, and I've asked him if she knows that we're together, and he says yes, but she doesn't appear to be backing off and it's scaring me. Am I being irrational? Should I just give it more time? He texts her (and other friends) back when he and I are snuggling or spending time together.