Okay lets get a few things straight:
1. Fergie never officially left the BEP but she isn't performing with them
2. BEP are set to make a comeback soon
3. BEP are having lots of different female vocalists perform in Fergie's place during their shows
4. will.i.am is a huge supporter of Kpop, specifically 2NE1/CL
this is a rumor posted by naver entertainment (where most kpop news is first broken) but the reporter doesnt really back up his claims, just his suspicions.
So, it seems that people are thinking CL will take Fergie's place - though its unclear whether its just for one performance or if its a more permanent gig (like, will she be part of the comeback or just part of the live shows)