For some reason one of my friends had the rule where, if you purchase a dragon card from the "river" and then you play that card. Then initiates another Dragon attack?
It's hard to tell, but are you querying the correctness of this rule? In which case, please refer to either the above post or the rulebook.
In case you posted this thread to draw attention to your friend's house rule, you should have posted under Variants (not Rules), and let us know why you liked it or not. (I suspect not).