If Kowalski is Fraser's "best friend," then what is RayV to Fraser?
And why would Fraser leave if RayV is still undercover? Life at the 27th didn't stop with Kowalski gone. Another officer would probably be appointed to be undercover as "Ray Vecchio," so Fraser would have to be seen as that officer's partner to continue to protect RayV's cover, even if Fraser is grieving for Kowalski. Kowalski isn't the ONLY person in Fraser's life, and Fraser should understand that and continue trying to keep RayV safe, if he still cares about RayV. Or maybe Fraser has become a selfish bastard and doesn't care about anyone anymore except Kowalski!