Even supposing of a friend, it is still problematic due to the connotations of affection.
From Oxford languages:
Affection: a gentle feeling of fondness or liking.
Suppose I say I have a gentle feeling of fondness or liking (i.e. affection) of a friend for my enemy. How would this make sense? I would be saying that the liking or fondness I have for a friend is the liking or fondness I have for an enemy. Metta has nothing to do with liking or fondness.
On noun vs verb. I’m not sure it matters too much once metta is translated as love. e.g.
- Have metta for all beings will be translated into
- Have love for all beings which, because English allows it, will eventually turn into the short-form
- Love all beings
This is another reason why translating metta as love is problematic.
The word metta or even the commonly used translation of loving-kindness, on the other hand, are generally not used as verbs. I.e. you would not say:
- Metta all beings; or
- Loving-kindness all beings
However, if one knew the difference between the noun and verb forms of love and one were careful to use the noun form and one knew Dr. Johnson’s specific definition of it then I suppose one could translate love as metta. But this is a rather unlikely prospect. The definition of words is largely a matter of what exists in the collective psyche, especially for a language that is currently in use and evolving. It doesn’t matter that love might have had a better definition decades or centuries ago. All that matters is that it doesn’t have such a definition now - i.e. it is not used in that way now by the masses.
Susan’s preoccupation with cats doesn’t automatically discount her metta. If she has metta and happens to be focused on cats, the metta will be directed towards the cats. However, we can devise a test to see if what she has is truly metta or something more partial.
Susan is currently focused on cats. But let’s suppose Susan comes across some termites and her focus shifts to them. Let’s further suppose that she understands that termites are sentient beings. If that feeling of ‘metta’ disappears on account of her focus shifting (i.e. the wish for the living beings’ welfare evaporates as she focuses on the termites), then the metta she had towards the cats would not have been metta at all, on account of it being partial. On the other hand, if the feeling of metta does not disappear on account of her focus shifting, then the metta she had for the cats was true metta.
To be fair, to have metta in an impartial way is difficult so it is likely that what most people believe is metta has some partiality to it in practice. However the further on the path to arahantship someone gets, the more this partiality could be expected to fall away.