Awkward...I don't see how you can remove the collar without talking to your neighbours. On the other hand, if you've already done a spot on flea treatment on her then the collar must be removed as you're doubling up on treatments which is a really bad idea. Remove it then talk to them (or get your mum to) is the best option. Removing it without talking to them could be counterproductive - yes, cats loose collars when they're out and about (I lost count of the number of collars I found adorning the bushes when growing up!) but for all you know they'll replace it and keep replacing. From what you said, you're not even certain that it's a flea and tick collar. Remove it, but keep it and talk to the neighbours is the best option. I know you don't want to talk to them, but it may be your only choice now. Get your mum to help if you can. God luck.