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I think the record supports the straightforward reading as the historic intent of this phrase.

Bartlett's Quotations lists:

NUMBER: 8527
AUTHOR: Plautus (c. 254–184 B.C.)
QUOTATION: Nothing is there more friendly to a man than a friend in need.
ATTRIBUTION: Epidicus. Act iii. Sc. 3, 44. (425.)

Note 1.
A friend in need is a friend indeed.—Hazlitt: English Proverbs.

A more complete reference would be:
p12; William Carew Hazlitt; English Proverbs and Proverbial Phrases, etc.; Reeves & Turner, Publishers; London; 1869 (2nd edition, 1882, it appears on p.14, as also in the 3rd edition in 1907, p.14.)
Note: in the first (1869) and second (1882) editions, you will see that "indeed" is rendered as two words "in deed." However, in the 3rd edition (1907), it appears as one word "indeed."
Hazlitt: English Proverbs, 1869
Hazlitt: English Proverbs, 1882
Hazlitt: English Proverbs, 1907
(Here Hazlit also notes: "The Spaniards say, Mas vale buen amigo, que pariente primo. — Ray.)

Clearly by the time of Hazlitt's writing of the first edition in 1869, this was already a well known saying, and even Hazlitt refers to it as "the old proverb" in his essay, "On Disagreeable People." Probably it is a clever loose translation of the Latin above which, in the original reads as:
Nihil homini amicost [amico est] opportuno amicius:
(sine tuo labore quod velis actumst [actum est] tamen.)

Other related quotes, sometimes thought to be in the historical line are:

'Amicus certus in re incerta cernitur'. (Quintus Ennius; c. 3rd century BC) [A true (cer-tus) friend is discerned (cer-nitur) when things are uncertain (in-cer-ta) (lit. "in an uncertain matter/thing").] This is quoted by Cicero, and attributed to Qunitus Ennius:

64. Itaque verae amicitiae difficillime reperiuntur in iis, qui in honoribus reque publica versantur; ubi enim istum invenias, qui honorem amici anteponat suo? Quid? haec ut omittam, quam graves, quam difficiles plerisque videntur calamitatum societates, ad quas non est facile inventu qui descendant. [emphasis mine] Quamquam Ennius recte: Amicus certus in re incerta cernitur; tamen haec duo levitatis et infirmitatis plerosque convincunt, aut si in bonis rebus contemnunt aut in malis deserunt. Qui igitur utraque in re gravem, constantem, stabilem se in amicitia praestiterit, hunc ex maxime raro genere hominum iudicare debemus et paene divino.
– Cicero; Laelius de Amicitia; 17.64

"It is sayd, that at the nede the frende is knowen." (William Caxton; Four Sonnes of Aymon, c.1489)

Prove thy friend ere thou have need; but, in-deed
A friend is never known till a man have need.
Before I had need, my most present foes
Seemed my most friends; but thus the world goes
(John Heywood's A Dialogue Conteynyng Prouerbes and Epigrammes, 1652)

And there are others, and other theories. Most such proverbs are commenting on how the mettle of one's friend is tested when one is in a time of need.

Happy to do it. It was interesting. In this case I would not place too much stock in the change from two words "in deed" to the one word "indeed." Spelling was still a little more fluid than it is now. The understood meaning is set by the author as the same as the Spanish proverb, so I qm confident the spelling change does not reflect a different interpretation, but rather reflects a shift in orthography.