Just ask him his views on the powerline adaptors, if he's had any problems with neighbours using them (or heard of anyone else having problems) and that you are aware that they can cause problems hence you're asking him first..
He'll either do one of three things
1# Give you a blank stare and close the door
2# Tell you there is/isn't an issue
3# have a good long chat with you and start telling you about ham radio whilst doing you a nice cup of tea whilst he demonstrates his equipment
Most will be #3, some though can be #1