Dog-gardening for THREE?

A cocoa-brown muzzle is snuffling beneath the east fence. It belongs to a dear old dauchsund that, apparently, has become a permanent or semi-permanent resident next door. (His human is dating my neighbor, and he appears to be living in the backyard there most days.)

P&B are delighted. They chase his shadow and sniff his muzzle and can't wait to see what he'll do next.

To accelerate this discovery, B-dog has begun digging a trough under the fence. In about 4 more inches our neighbor will have no trouble squeezing over for a visit. At which point I fear my gentle companions may attack him. (Once when he ambled over to our front yard they became downright hostile. Not sure why.)

The humans next door are nice people but don't quite get the dog-in-the-backyard-all-day problem, and since they witnessed P&B's aforementioned bullying it seems doubtful they'd interpret my suggestion that they reinforce their fence as anything other than a sign that my dogs are bloodthirsty devils. Which is the sort of expectation that leads to weirdly self-fulfilling prophecies.

Seems to me I have four basic options:

• keep my dogz inside until the romance ends next door, or until the dog makes his way into my yard so I can demonstrate the problem to the neighbor
  
  
• talk to the neighbor about sharing the cost of reinforcing both sides of the fence with a concrete berm (which seems really unlikely both because of the dog-obiviousness issue and because she declined an invitation to share expenses for fence preservation a year ago)
  
  
• do the concrete berm thing myself on just my side of the fence
  
  
• build a 40-foot-long raised planting bed against the fence

Guess which one I'm contemplating?

Hope it's a sunny weekend.