Making good DMPCs evil

[warlord]

In a game I'm running a major NPC ally of the PCs has just gone evil. My DMPC is romantically involved with her in the "your my world" kind of way. So how can I justify and more importently roleplay his trasition to evil?

 

[KingOfChaos]

How powerful is this evil NPC? Are they a spellcaster? You kinda left some things out that could possibly help us come up with something for your game.

 

[warlord]

real strong, real fast female fighter. She just started working for the BBEG the group is fighting. Does that give you need? if not I can explain some more.