MY take on it is:
Friend not happy with her life. Being a SAHP is hard. She wants the relationship to end.
If she stays in the home and DP leaves, how does she get back into work? It's tough out there for everyone. X number of years out of the workforce means that unless she's got some in demand skills, she's going to hard to find a job that fits with looking after DS. So she's now a single parent, still frustrated with her life.
If she leaves and only has to fend for herself, she will find it much easier to get a job.
OP says DP is a good man, so no worries that the child will be adequately cared for.
The Friend could be taking the long term/pragmatic view which doesn't necessarily dovetail with being a full time SAHP right now.
The way I see it see it is her options were:
- maintain status quo - stay in a marriage she doesn't want to
- DP leaves - but her life doesn't actually improve as highlighted above it would be hard (although not impossible) to get back into the workforce at a feasible level
- she leaves, gets on her feet. DS has adequate childcare provsion in place.
I can see why she has taken option 3.
Would I do it? I don't think so but then, I've never felt isolated/restricted/frustrated as I kept my career going. That had its drawbacks too as I worried about was I damaging the kids by not being a SAHP and all the usual guilt trade offs. I also think as a parent, sometimes you do have to suck it up and think "well my life's not ideal, but I'm not the priority here" - although there is always the balance between that and preserving your identity and sanity.
I don't think it's an easy situation to sit back and judge and although you know her, you're not living her life, feeling her feelings. Admitedly there is a chance she's just thought "ah balls to it, pass my Heat mag and a Dairy Milk, this is the life" but really, how likely is that?