TL;DR: My colleague receives way too much work and intends to work on holidays, without realizing the problem let alone airing it to management. How can I convince her to discuss with her manager?
"My manager gives me too much work" has already been asked a zillion times here and I could point her to any such post of those, but I feel there must be a more diplomatic way to bring it up.
Super-detailed background
I work for the R&D subsidiary (a few hundreds employees) of BigCorp (>100k employees), both of which are financially sound (AFAICT) and do not suffer from generalized workaholic culture. We are basically a subcontractor for BigCorp's other subsidiaries: they pay us to solve scientific problems they cannot solve of their own. The subcontracting unit is called a "study" - a study will typically last 1 year, involve 1-2 persons on our side and 2-5 on the client side; each employee on our side handles typically 3-4 studies in parallel (maybe 2 for project managers/high-commitment matters, 5 for more simple stuff). Cultural/legal background of every relevant entity is France.
My colleague is actually an "office roommate": we share an office but are assigned to entirely different teams with different expertise areas, management etc.
She is a recent hire, still under probation period. She was hired after her PhD so it's her first "real" workplace position. She is foreign-born (Balkan country), but has been in France for the last 7 years and has recently acquired citizenship; she speaks perfect French, is aware of all common cultural tropes, has more friends here than in her home country, etc.
While she has quite a stellar résumé and could easily find another job, she no money left at the moment and I believe that stresses her out into avoiding firing at all costs.
The problem
My colleague is overburdened with work. She is currently tasked with 4 studies in parallel, but with the lead role in each, which would already be on the higher end for established engineers (let alone junior hires that are still onboarding). She told me her manager is considering adding two more studies to her workload, with nontrivial responsibilities in both.
A rough guesstimate (since I do not know the details of the projects) is that she would be doing the work of two people. It kinda matches with my knowledge that her team had some recent turnaround and they still have one vacant position after hiring her.
However, her reaction is a mixture of fatalism and impostor syndrome. She told me she intends to work on holidays to make up for the extra workload, otherwise her boss will realize they should have hired someone else. She puts in quite long hours already, and quite a lot of that time is in meetings (*).
I told her that she should under no circumstance work on holidays and should speak to her manager. She was walking away to go for lunch and did not reply, but she seemed shocked at the idea of giving her manager feedback regarding her workload.
Question
How can I convince her that she is not the problem and needs to talk to her manager about her workload? Ultimately it's not my business what she does, but I want at least to give her my read of the situation in a way she can understand.
I considered reaching out on my own to her manager, or HR, or union representatives, but I cannot see a scenario where it helps if she is convinced there is no problem.
(*) It is possible to be bad at core areas of her job (experimental design, bibliographic studies, planning coordination etc.), but there is no way she can be really bad at sitting in a meeting and listening. She spends about 2/3 of a normal work week in meetings, which is again unusual - in a similar position I spend about 1/5. She is rarely the initiator of such meetings, but is still required to attend as the project lead.
