Don't give it flea treatment - it may already be getting something from its owners, or they may just be a bit late this month. Double dosing would potentially harm the cat.

Giving it the odd scrap is fine if you've checked with the neighbours, but anything else is overstepping the bounds. Cats are very good at making themselves appear neglected and in need of extra food and love. It'd be like neighbours offering your kids lollies every time they saw them without you around.

Talk to the neighbours and establish a relationship. It's their cat, best to leave it at that.