flu, I am pretty hopeless at these things in practice. As is often the case, however, the most clueless are often the first to volunteer advice, so here goes…
How about setting up some discussion with you, your neighbor, and a landscaper, so the landscaper can discuss all the pros and cons of various plants and HOA rules as if they were just giving helpful advice to two good neighbors with no gardening or other issues between them? For example, tell your neighbor that it’s great that he’s making changes, because you just happen to be talking to a really smart and helpful landscaper about what works and doesn’t work on your own lot. Ask your neighbor if he wants to sit in when the landscaper comes over next. You like your neighbor so much (you tell him) that you can just set it up so you’ll all be there, and have a few drinks and talk plants.
Just make sure the landscaper is the right guy who says the right things!