STYLETHREAD -- LET'S TALK SHOP!

It was probably because it just wasn't was she was expecting to happen, because it's never happened before!  Dogs are very specific learners, so even though he knows this guy is okay when you're around he wasn't sure if he was okay when you weren't around.

Next time he comes over (with you there), have him give Fala some treats and some attention.  If he seems fine with him you and hubby can try stepping outside (or going upstairs or whatever) and leaving them alone for a few minutes (lots of treats while you're gone!).  Come back for a few minutes, leave for a few minutes, come back.  If he's handling it well (no aggression toward your friend) make a bigger production of leaving, out the front door like you're really going somewhere.  Stay out for a few minutes and come back.  Then have your friend go outside, you guys give some treats and love, and then you leave.  A few minutes later, have your friend go back in by himself, armed with lots of treats.  If he's good, when you guys come back a few minutes later make sure to give lots of praise.

The idea is to teach Fala that this guy is always okay - if he's here when you are, if he comes and then you leave, if he comes when you're already away, etc.  It can be spread over time and done in smaller chunks, depending on how quickly he picks it up.  Once you've built the trust up a bit I doubt Fala will take such issue with him coming over while you're away.  Good luck!