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In indo-european folklore the common expression "the same in happiness and sorrow" usually means one who stays a loyal friend not only in happy times but also in times of trouble, when having all responsibilities of a true friend could be taxing. This corresponds with #3 on the list, "in misfortune he does not forsake one".
By extension, "the same in happiness and sorrow" means one who is not seeking a benefit for oneself from the friendship, but sincerely gives oneself away to the friend. This is where the other three come from:
(i) he reveals his secrets,
(ii) he conceals one's own secrets,
...
(iv) his life even he sacrifices for one's sake.
"Warm-hearted" means a friend whose relationship of trust and commitment comes from the heart, is unforced and natural. Such friend would not have to make an effort to "sacrifice his life for one's sake", it would come naturally.
Altogether, the above is not to be understood literally, but is meant to portray the attitude of uncalculated selfless fondness, almost love, as the perfection of friendship.
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The last item, "his life even he sacrifices for one's sake" -- can that be reconciled with the answers to the question, How to be compassionate to a friend in need? Those answers seem to suggest that you can only help yourself.
Apparently the question in the OP was mistaking the purpose of a "friend": if you are wise your friend isn't there to help you (i.e. doesn't exist only to help you), although, they do help you; instead your friend is there (i.e. is with you) so you can help them.
Thus spoke the Exalted One. And when the Master had thus spoken, he spoke yet again:
The friend who is a helpmate,
the friend in happiness and woe,
the friend who gives good counsel,
the friend who sympathises too —
these four as friends the wise behold
and cherish them devotedly
as does a mother her own child.
If you're wise you will cherish your "friend in happiness and woe" and (as does a mother her own child) keep them from the harm. Another translation says, instead of "cherish", "attend on them carefully".
In other words if someone is your friend then you will cherish them.
Not everyone is (defined as) your friend:
These four, young householder, should be understood as foes in the guise of friends
The Buddha does not recommend that you cherish these:
these four as enemies the wise behold,
avoid them from afar as paths of peril.
It's not clear how (or by whom) such a "foe" can ever be helped or cherished.
In summary if someone is a genuine friend (helpful, constant, good advice) then cherish them; if someone is a false friend (costly, a friend in word but not deed, a flatterer with bad advice, dissolute) then avoid them as perilous.