I'm willing to bet that your betta isn't really lonely. bettas like attention. Mine goes as far as not moving at all if i so much as talk to my cichilid. I only got a tank-mate for my beta, because i had a 20g and i got tired of hearing my boyfriend make fun of me for having a single fish in a 20g tank. *sigh* Since it's a 5g, I don't think that you really need anything else in it other than your spoiled beta! I say spoiled in the most respectful way, of course. ;)
Maybe add some real or silk plants? More hiding places?
Now that my opinion is out of the way, I can tell you more about tank-mates incase you get a larger tank later on down the road (don't rush it. Your betta is your baby and they know it!). I answered a question that someone was asking regarding their 10g: http://www.bettafish.com/showthread.php?t=126971