The best time to prune most trees growing in temperate climates is in mid winter when the tree is dormant and the sap is not flowing (to the best of my knowledge, sap does not flow year round in walnuts). Trimming now when the tree is starting to break dormancy and sap is flowing on warm days will cause weeping of sap at the wound. This invites diseases and insects to infect the open wound. I would ask your neighbor to be patient and trim the tree at the appropriate time of year - mid winter next year.

Walnuts should be pruned while in leaf, otherwise it bleeds like there is no tomorrow. Most other trees do get pruned while bare, that also helps with seeing what you need to cut, but walnuts of all stripes should be in leaf.

Dzitmoidonc - could you pass along a reference for that? I have always heard that they should be pruned during dormancy in winter like other trees. I'm not being argumentative or saying "prove it" - I just would like to learn more if I was misinformed!

Somebody on here (Lucky?) told me years ago that walnuts should be pruned in the summer. I didn't listen the first year and pruned a Juglans angustifolia in the winter. It still has a large black spot where the sap ran down. Since then, I wait until leaf out and prune. Same for the Butternut etc. I suspect those who advocate winter pruning have never actually done it.

This from drclarkia.com:
Any pruning should only be carried out in late summer to early autumn or when the plant is fully dormant otherwise wounds will bleed profusely and this will severely weaken the tree.

This from the Royal Hort. Society: Juglans (walnut): Prune mid-summer, before mid-winter.

I'm sure lots of other sources are available. Like I say, my experience often teaches me, and stars like Lucky and Guy Sternberg, Brandon and Beng and others have helped guide me greatly through the years.

Thanks Dzitmoidonc. I'm glad to know I wasn't too far off recommending mid-winter pruning. I'll have to adjust that to "prune just before mid winter" while the tree is still dormant OR in mid summer after the bulk of the sap flow has ceased. Much appreciated. Now that I think about it, that does make sense. I have some 'valley forge' and 'new harmony' american elms that I prune annually to help them reach their ideal tall vase-shaped canopy. I prune them in January, and then tip-prune the new growth mid to late summer.