It's a bit of a gray area. Implied status as a worker is regulated by R186(u)
http://laws.justice.gc.ca/eng/SOR-2002-227/page-5.html#codese:186
186. A foreign national may work in Canada without a work permit
...
(u) until a decision is made on an application made by them under subsection 201(1), if they have remained in Canada after the expiry of their work permit and they have continued to comply with the conditions set out on the expired work permit, other than the expiry date.
R201(1) says this:
http://laws.justice.gc.ca/eng/SOR-2002-227/page-5.html#codese:201
Application for renewal
201. (1) A foreign national may apply for the renewal of their work permit if
(a) the application is made before their work permit expires; and
(b) they have complied with all conditions imposed on their entry into Canada.
Renewal
(2) An officer shall renew the foreign national's work permit if, following an examination, it is established that the foreign national continues to meet the requirements of subsection 200(1).
Implied status as a worker is usually where someone on a restricted work permit gets a new LMO and applies to extend their work permit. In that case, they have the LMO and proof they've applied, and they're working for the same employer who knows the situation. In that case, the employer knows they're still eligible to work.
Now, the application for the work permit you submitted with the PR application will be approved once approval in principle is reached, as it meets the requirements of R200(1) which is all standard work permit stuff. However, the ambiguity comes from whether or not that WP application really counts as a renewal, or whether it's actually an application for a new work permit. That really isn't clear in the regulations. If it does count as a renewal - as I suspect it does - then the regulations give him implied status as a worker, under the conditions of the existing (open) work permit. If it does not count as a renewal, then most likely his implied status is as a visitor, not a worker.
Implied status as a visitor is specifically addressed in the relevant processing manual, IP6. Implied status as a worker is not.
http://www.cic.gc.ca/english/resources/manuals/ip/ip06-eng.pdf
5.6. Inland applications for permanent residence
In situations where an applicant who has visitor status submits an application for permanent
residence to Vegreville and at the same time submits an application for a work permit (pursuant to
R207(b)), the visitor may be considered to have requested an extension of their TR status (in
accordance with R183(5)). They are considered to have implied status as a visitor, until a
decision is made on their WP application.
When no application for a work or study permit is received with the application for permanent
residence, the applicant is obliged to apply to extend their visitor status.
However, in practice, since his temporary SIN card will expire the same time his work permit does, even assuming it does count as a renewal, and his implied status is as a worker, he will have nothing to prove he's actually entitled to work. So, while my understanding is that he can continue to work legally, it will be difficult to get work with a new employer. If the employer is accommodating, it's possible, but employers are instructed to not employ people without a valid SIN.
To answer your specific question about employers, though - if he's still able to work at all, it's for any employer.
Personally in that situation I'd carry on working, as there's a good argument to be made that implied status is as a worker, and under the conditions of the existing work permit. I'd be very surprised if that ended up causing problems, and even if it did, the inland spousal public policy means the PR app would still go through even if they were to decide he'd worked illegally.
One other thing is important to note, and that's that if he leaves Canada and re-enters, he definitely no longer has implied status as a worker. That's specifically addressed by OP11 section 24:
http://www.cic.gc.ca/english/resources/manuals/op/op11-eng.pdf
