I bet he hated Boris.
IS hated everything that shifted Helen's interest away from himself. I don't get the feeling that he had any regard for all that Helen loved: her gifts in writing, her compassion towards others, love of aesthetics, 'quiet' time of reflection, loved of Artisan cheese - liking of beautiful jewellery - her love of London. He disregarded everything, trampling on any of her choices whilst maintaining his OWN.
I don't know whether he likes or dislikes dogs - but I certainly believe he would be jealous of Boris, because Boris was part of Helen and JS's life - and Helen adored her baby dog. Boris and Helen spent deep companionship together - and Boris was a reason for Helen to get up in the morning during her treacherous grief of John S.
That would be 'all too much' for this narcissist's genuine tolerance of Boris. And he wouldn't be able to begin to understand the love Helen had for her precious Dachshund.
If HE understands love at all.