This sounds like a recipe for a messy divorce. It makes sense for a bank to offer a discounted mortgage, because they're in the business of mortgages, and it's not unreasonable to still bank with a bank you used to work for.
OTOH, I wouldn't want to have to have a payment relationship with my employer after I left. Sometimes it's nice to keep a 401k with an old employer, but you end up caught up in their drama still --- when they change 401k providers, you need to pay attention, if they go through a merger, you need to pay attention, etc. It would be worse if they were administering your mortgage.
Now --- if they want to partner with a bank/credit union and subsidize a lower rate or something, that might make sense. It could be win-win-win; it might be less risky loans for the bank, a nice perk for the employee, and a way to attract employees. Of course, it depends on what happens the employer; it also concentrates the loan portfolio of the bank; if you had a lot of loans to employees of a company that has a big problem, you've now got a lot of borrowers risking default at the same time. I got a nice deal on a car and factory loan incentives because of who I worked for, and there's lots of other employee perks available.