can't be "he" because, absent a possessive (his), being is a participle/adjective, hence the noun is "he" or "him", but it's ruled by "on" so has to be "him"

"her and Angel's" ISN'T wrong: but i think it feels wrong because of the mismatched shapes of the possessive endings? you feel you want to say "hers and Angel's", which is DEFINITELY wrong and feels somehow corrected and tidy and symmetric