They're supposed to be good tank mates from what I've learned, but my experience hasn't been entirely favorable.
When I first got Darby (my Betta) he was in a 10g with 2 ADF's and a Mystery snail. The first couple of days were fine, everyone going about, doing their own things. After a couple of days though, Darby started getting curious about his tank mates. I noticed him following the ADF's, watching them carefully. Then he moved to my mystery snail. It was a lucky thing I was watching, because he snapped off poor Bento's antenna! I was shocked because I'd heard nothing but good things about snails with bettas.
After he got a taste of Bento, he started chasing the frogs around and I just decided enough was enough and got a 3g for Darby to sit on my computer desk. Their little toes DO look slightly like bloodworms, so I didn't want to take that chance.
You never know though, I've read others having good experiences with ADF's and Bettas. They're all different personality-wise. Just be prepared if your guy decides to take a nibble. Considering the damage Darby did to Bento's shell, I'm sure they could take a sizable chunk out of a soft little ADF.