Not sure how helpful this reply is going to be, but I find Dragon to be the worst of all the expansions. I have all Talisman 4th Edition expansions and I was a playtester for Cataclysm. I love Cataclysm and I find it plays very well with all the other expansions, except Dragon. But not because the two don't mix specifically, just that Dragon doesn't mix with anything, IMHO.
Dragon just adds too much "other stuff" you have to do each turn. Especially when combined with the Reaper, Blood Moon and Harbinger, it just becomes too much and the board gets saturated with Dragon tokens.
That being said, if your friend really enjoys the Dragon expansion I'm sure he'll love combining it with Cataclysm. Now is a good time to get any and all expansions given FFG's recent split with Games Workshop; it could be a long time (if ever) before we see any more Talisman games being printed. Supply will go down and prices will go up.
Funny you mention that.
My 3 friends combined have: Dragon, Reaper, Harbinger and the City
I think they like messy boards