My Best Friend's Marriage

My best friend's marriage is stressing me out.  Seriously.  They've been married for just over a month and are the most wonderful couple, but must live apart right now due to his current military service, and she seems to be stressing really hardcore.  They love each other, and pretty much all he talks about lately is how much she makes him happy, he loves her, can't wait to see her, etc...they talk at least twice a day, and he is very devoted, even flying to see her every few weeks when possible.  But the last few nights he's expressed real fear over her irrational behavior, and I'm starting to get nervous for him.  I'm not married, and can't profess to know anything about anything, but I care deeply for them both and he is my very.best.friend, so it worries me.  I don't know what to say...I think perhaps she is depressed and doesn't realize it because I've always known her to be logical and rational, but she seems distant in our friendship lately, and what he describes sounds a lot like mood swings, irritability, and hostile attribution bias.  So either she's pregnant or depression is creeping in...either way, it's difficult for me to watch it play out knowing there's nothing I can do.  How do people conquer these things?