Rob Kardashian: Blac Chyna's Preying On Him ... Friends Say

Rob Kardashian is in a vulnerable place right now and Blac Chyna not only knows it, she's taking full advantage ... according to people in Rob's inner circle.

Sources tell us Rob's close friends and family members tell us he's dealing with so much right now -- his ongoing weight issues and his recent diabetes diagnosis -- and they think BC made her move now, because his guard is down.

We're told they think Chyna really only wants 2 things: publicity and to get back at Kylie Jenner for hooking up with BC's baby daddy, Tyga.

Our sources say Rob's peeps think the fact Chyna's BFF Amber Rose just started following Scott Disick on IG ... is more proof this is a full scale "thirsty" attack.

They're all for Rob "having some fun" -- but they just hope he sees BC for what she is.