Is 'stewer' a proper English word?

'Stewer' is a proper English word, formed by attaching the agentive suffix '-er' to the verb 'stew'.

-er, suffix¹
....
2. ... In modern English they [the -er derivatives formative of agent nouns] may be formed on all vbs., excepting some of those which have agent nouns ending in -or, and some others for which this function is served by ns. of different formation (e.g. correspond, correspondent). ... The agent nouns in -er normally denote personal agents (originally, only male persons, though this restriction is now wholly obsolete); many of them, however, may be used to denote material agents, and hence also mere instruments; e.g. blotter, cutter, poker, roller, etc.

["-er, suffix1". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/63869?rskey=3VncJU&result=5&isAdvanced=false (accessed March 02, 2016). Bold emphasis mine.]

Although individual entries for 'stewer' in the sense of 'stewing pot' do not appear in the dictionaries I checked, that failure to appear is not unusual for comparatively rare words formed with the agentive suffix '-er'.

The earliest uses of 'stewer' that I could uncover, in the material agent noun sense used by your mother, appeared in advertisements in the popular press in the mid-1800s. Here is a representative example from 1854 (Wheeling Daily Intelligencer, June 20, 1854), advertising the availability of an 'oyster stewer' from a purveyor of planished (smoothed) tin wares:

(Ad is in the third column, eighth from the bottom.)

While the commercial context of early uses of 'stewer' in the sense of 'stewing pot' might seem to suggest that the origin of the word can be traced to advertisers and manufacturers of cookware, it is more likely that those advertisers and manufacturers adopted and used a word that would be recognized immediately, because it was already in common use, by people wishing to purchase the item.

Appearances of 'stewer' with the sense of 'stewing pot' continue throughout the years covered by the corpus of popular press available for a search. For example, a search for 'stewer' at "Chronicling America", an online archive of digitized US newspapers covering the years 1836-1922, returns approximately 6600 (this number includes false hits) newspaper pages containing the word 'stewer'; of those pages, at a guess based on a close examination of random samples, approximately 40-60 percent contain the word 'stewer' used in the sense of 'stewing pot'.

Thus, notwithstanding the failure of lexicographers to collect the word, or at least to add it to dictionaries, 'stewer' is well-documented in use through 1922. It beggars belief that in 1923 the word suddenly fell out of use, but I would not be altogether surprised if 'experts' who are inclined to suppose, despite the smothering weight of contrary evidence, that their 'expertise' encompasses the entirety of the 'proper' English lexicon might also be inclined to suppose that words suddenly and mysteriously vanish utterly.

One reason for the comparative rarity of the material agent noun 'stewer' may be that other words with similar senses were and are in common use. Those words include the still extant compounds 'stewpot' and 'stewpan', and the earlier 'stew', used from the early 1300s through at least the mid-1600s with the material agent noun sense, a sense that is now obsolete for this form of the word. For example, 'stew' in the sense of 'stewpot' appears in Shakespeare's Measure for Measure:

Where I have seen corruption boil and bubble
Till it o'er-run the stew; ....

Instances of the use of the personal (as opposed to the material) agent noun 'stewer' appear in the early 1800s in the Google Books corpus. For example, there is a reference to an "Herb stewer", one member of the "King's Household" in The Weekly Register of June 5, 1813. Appearances of the personal agent noun are somewhat more common during the 1800s than the appearances of the material agent noun with the sense 'stewing pot'.

"Stewer" certainly comes from "stew" but it's not in any dictionaries I've consulted with the meaning of crock-pot. If that's your standard of propriety, then this is not a "proper" word.

But notice that there is a convention of adding the "-er" ending for appliances. Consider "cooker," an appliance for cooking food (here).

Also,

• "fryer"
• "peeler"
• "grater"
• "strainer"
• "toaster"
• "roaster"
• "steamer"
• "melon baller"
• "potato ricer"

This convention of using "-er" to form a name for an appliance is an example of a productive morphology. Most speakers would recognize that it's acceptable to add "-er" to a word to form the name of an appliance related to what the original word denotes (for example, "stew" → "stewer").