It might be as simple as Digby simply not liking this guy and being a bit vocal about it.
My best advice would be if that friend visit again to give Digby a favorite toy and putting him outside or in a part of the house where he can't see the ''intruder''. I realize it's not an ideal situation and definitely not ideal to help Digby being a more social dog but I think in this case it's best for everyone's sanity (and hearing). If you have to put him outside during such a visit make sure you're feeling utterly calm and confident and absolutely do not make any excited noises or scold him (speak as little as possible and in a low voice). Dogs are masters at telling our true feelings and if you give Digby any negative energy he's going to throw it right back at you.
Perhaps other more experienced owners would have better advice.
My best advice would be if that friend visit again to give Digby a favorite toy and putting him outside or in a part of the house where he can't see the ''intruder''. I realize it's not an ideal situation and definitely not ideal to help Digby being a more social dog but I think in this case it's best for everyone's sanity (and hearing). If you have to put him outside during such a visit make sure you're feeling utterly calm and confident and absolutely do not make any excited noises or scold him (speak as little as possible and in a low voice). Dogs are masters at telling our true feelings and if you give Digby any negative energy he's going to throw it right back at you.
Perhaps other more experienced owners would have better advice.