2. If your friend wants part-time work as an employee, they can try negotiation. It's harder to negotiate for new position, but can be done. I've negotiated less than full time at multiple companies, for example. E.g. I interviewed someone who has been doing 4 days a week for 15 years now: https://codewithoutrules.com/2018/01/08/part-time-programmer...
3. There's contract work, where you sign contract for 6 months or whatever. Randstaad and Winter Wyman advertise those. I have no direct knowledge, but my guess is it's easier to negotiate shorter workweek with those, since it's already a staff augmentation kinda thing.
Easiest thing for your friend in particular to do is probably get in touch with previous employers (or previous managers/co-workers who have moved on to other companies). Those companies/managers will already know your friend, and so be willing to give them a try.
Broad overview of options: https://codewithoutrules.com/2018/07/03/35-hour-workweek/