She is identified in CP's Scales article (Vol  XI, p. 503 and note (c) as
(1) the daughter of John de Northwood,
(2) the Northwood family was of Kent,
(3) She died in 1414.
(4) CP says, XI, 501, her best pedigree is in Arch Cant, vol ii, pp. 14, 36, 37.

Testamenta Vetusta has the will of a John de Northwood, p. 102 which includes:
(1) A wife Joane
(2) A daughter Joane
(3) A note by the editor saying John de N. was the son of Sor Roger Northwood by Juliana Say, dau of Baron Say.
(4) The note also said John m. Joan Hert, dau of Robert Hert,

Test Vet also includes the wills of this Joan Northwood, p. 184-5, her first husband Roger Scales on p. 120, her son Robert Scales on p. 151.  But none of these give any more information.

Then in the Northwode article, CP Vol IX, p. 755, CP says that there was a John de Northwode, 3rd baron who m. Joan Hert and d. in 1378/9.

My assertion is that this Joan's father, John de Northwood and CP's John de Northwode, 3rd baron, are the same person.  Nicolas in Test Vet certainly made the same conclusion, p. 184, note (1).
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In her will, dated December 29th 1414, she lists:
Edmond Thorp, Knight, her husband
Robert Scales
Lady Katherine Savage [nee Scales],
Her daughter Isabella [Thorpe],
Lord Scales, her late husband,
Her daughter Lady Joan [Scales],
Her daughter Joan [Thorp].