"Bros before hos" & "All's fair in love & war" are equally applicable in this case up to certain limitations. This situation is really predicated on Boris's relationships w/ Adam & Amy when they all met. If Boris was indeed a friend of Adam before they broke up then his actions are definitely shady. But, if they were friends after, then there is no priority there. Boris is not obligated to Adam because they share no past history so he is free to pursue Amy. Now, even it was revealed to him in the 2nd scenario that Adam & Amy had a previous relationship, that has nothing to do w/ Boris so Adam has no right for concern.
Now, if Boris had a previous relationship w/ Adam then the only legal action he can take to pursue Amy is by abiding by the half-life rule of the Man-Law or its exception. He's got to wait for either a 'mourning period' equal to half the duration of Adam & Amy's relationship or a maximum of 6 months (whichever one is shorter) before any pursuit is legally allowed. The reasoning for this is that friends have to respect where others have been but that 6 months is enough time for any average guy to get over it and find someone new.
When a couple break up, they lost the privileges (as well as the drama) that come w/ the relationship, the only remaining connection between them is emotional and emotions are rarely logical creatures.