it's not for you to find this out, it's for your friend, and I'd avoid trying to warn your friend, because it will only drive you apart, let him discover this himself if there is anything to worry about.

Some people fall for each other straight away, and when that happens, you feel comfortable around them and opening up to them. Sounds like they are going to fall in love, and he's probably not going to believe other people's suspicions about his new girlfriend, he has to find that out himself. If two people want to be together, they will be regardless of friends and family members.